package app

import (
	"context"
	"errors"
	"fmt"
	"io"
	"os"
	"os/exec"
	"os/signal"
	"strings"
	"sync"
	"sync/atomic"
	"syscall"
	"time"

	cpty "github.com/creack/pty"
	"golang.org/x/term"

	"github.com/hjbdev/patterm/internal/mcp"
	"github.com/hjbdev/patterm/internal/persist"
	"github.com/hjbdev/patterm/internal/preset"
	"github.com/hjbdev/patterm/internal/scratchpad"
	"github.com/hjbdev/patterm/internal/trust"
	"github.com/hjbdev/patterm/internal/vt"
)

// Options configures a patterm run.
type Options struct {
	ProjectDir string
	ProjectKey string
	// DebugDir, when non-empty, enables verbose debug logging to
	// <DebugDir>/patterm.log and per-child raw PTY output capture to
	// <DebugDir>/<child-id>.raw. The dir is created if missing. Events
	// (spawn / exit / state change) land in <DebugDir>/events.jsonl.
	DebugDir string
	// ProfileDir, when non-empty, enables in-process performance
	// counters. patterm writes a per-second JSONL snapshot stream to
	// <ProfileDir>/metrics.jsonl, a final aggregate to metrics.json,
	// and a human-readable summary.txt on shutdown. The pprof files
	// written by --profile sit alongside these in the same dir.
	ProfileDir string
}

const keyCtrlK byte = 0x0b

// Run is patterm's single-process entry point. SPEC §2: one Go process
// owns everything; no daemon, no detach, no socket-based reattachment.
func Run(ctx context.Context, opts Options) error {
	if opts.ProjectDir == "" {
		return errors.New("app: ProjectDir required")
	}

	presets, err := preset.Load()
	if err != nil {
		return fmt.Errorf("app: load presets: %w", err)
	}
	appSettings, settingsPath, err := loadSettings()
	if err != nil {
		logf("settings load: %v", err)
	}

	// Ensure the per-project scratchpad dir exists so MCP and the UI
	// can read/write into it. SPEC §3.
	pads, err := scratchpad.Open(opts.ProjectKey)
	if err != nil {
		return fmt.Errorf("app: scratchpad init: %w", err)
	}

	// Per-project trust store for command-preset trust gating (SPEC §7).
	trustStore, err := trust.Open(opts.ProjectKey)
	if err != nil {
		return fmt.Errorf("app: trust init: %w", err)
	}

	// Per-project persisted-process store. Survives across patterm
	// restarts so user-created top-level command processes come back
	// after a relaunch.
	persistStore, err := persist.Open(opts.ProjectKey)
	if err != nil {
		return fmt.Errorf("app: persist init: %w", err)
	}

	// In-process MCP server bound to the per-PID socket. Children that
	// support MCP get pointed at `patterm mcp-stdio --socket=... --identity=...`.
	// SPEC §10.
	mcpSrv, err := mcp.Start()
	if err != nil {
		return fmt.Errorf("app: mcp start: %w", err)
	}
	defer mcpSrv.Close()

	sess := NewSession(opts.ProjectDir, opts.ProjectKey)
	defer sess.Shutdown()

	// Debug capture: when --debug=<dir> is set, write a verbose log
	// (patterm.log), per-child raw PTY output (<id>.raw), and a
	// JSONL event stream (events.jsonl). Installed before the TUI
	// listener so the very first OnChildSpawned / OnPTYOut event
	// is captured.
	if opts.DebugDir != "" {
		dc, err := openDebugCapture(opts.DebugDir)
		if err != nil {
			return fmt.Errorf("app: debug capture: %w", err)
		}
		os.Setenv("PATTERM_DEBUG_LOG", dc.LogPath())
		sess.Subscribe(dc)
		defer dc.Close()
		logf("debug capture enabled at %s", opts.DebugDir)
	}
	// Snapshot persisted processes BEFORE attaching the store: Spawn
	// mints fresh ids, so the old records would otherwise linger
	// alongside the new ones. Drop them up front; the restore loop
	// below re-saves each entry under its new id.
	savedProcesses := persistStore.List()
	for _, e := range savedProcesses {
		_ = persistStore.Remove(e.ID)
	}
	sess.SetPersistStore(persistStore)

	cols, rows := hostSize()

	layout := newTerminalLayout(cols, rows)

	// Launcher handles preset → child translation, including MCP
	// config injection for agent presets.
	launcher := NewLauncher(sess, mcpSrv.Socket(), layout.childCols(), layout.childRows())

	// Wire the tool host into MCP. Spawns through MCP use the host
	// terminal's viewport grid for their initial PTY size; SIGWINCH paths
	// resize them later.
	host := newToolHost(sess, pads, launcher, presets, trustStore, layout.childCols(), layout.childRows())
	mcpSrv.SetHost(host)

	var restoreState *term.State
	if term.IsTerminal(int(os.Stdin.Fd())) {
		st, err := term.MakeRaw(int(os.Stdin.Fd()))
		if err != nil {
			return fmt.Errorf("app: stdin raw: %w", err)
		}
		restoreState = st
	}

	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	// Performance tracker — instrumented hot-path timings written to
	// <ProfileDir>. nil when --profile is off, in which case every
	// record*() call is a fast nil check.
	metrics, err := newMetricsTracker(opts.ProfileDir)
	if err != nil {
		return fmt.Errorf("app: metrics tracker: %w", err)
	}
	if metrics != nil {
		go metrics.run(ctx)
		defer metrics.close()
	}

	// Per-session idle-detection classifier. One goroutine ticks every
	// 250ms over every live child and updates IdleState. It stops when
	// ctx is cancelled.
	go sess.runClassifier(ctx)

	st := &uiState{
		sess:         sess,
		presets:      presets,
		launcher:     launcher,
		pads:         pads,
		chromeWake:   make(chan struct{}, 1),
		trust:        trustStore,
		timers:       host.timers,
		hostCols:     cols,
		hostRows:     rows,
		stdinTTY:     term.IsTerminal(int(os.Stdin.Fd())),
		metrics:      metrics,
		settings:     appSettings,
		settingsPath: settingsPath,
		ctx:          ctx,
	}
	st.summaries = newSummaryManager(sess, opts.ProjectDir, presets, func() autoSummarySettings {
		st.settingsMu.Lock()
		defer st.settingsMu.Unlock()
		return st.settings.AutoSummary.clone()
	}, func() {
		st.markChromeDirty()
		st.markSidebarDirty()
	}, func(_ string, result summaryState) {
		if result.Error != "" {
			st.flashError(fmt.Sprintf("summary: %v", result.Error))
		}
	})
	sess.SetMetrics(metrics)
	host.attention = st
	host.focus = st
	host.prompter = st
	host.scratch = st
	st.lastExit.Store(-1)
	sess.Subscribe(st)
	go st.summaries.run(ctx)

	st.enterScreen()
	st.renderEmptyState()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()

	// Set initial PTY grid for any future child. The child gets the
	// computed main viewport, excluding tab bar, sidebar, and status.
	sess.ResizeAll(layout.childCols(), layout.childRows())
	launcher.SetSize(layout.childCols(), layout.childRows())
	host.SetSize(layout.childCols(), layout.childRows())

	// Replay persisted top-level command processes. Failures are
	// logged and skipped so a stale entry (preset deleted, binary
	// missing) doesn't block startup.
	for _, e := range savedProcesses {
		c, err := launcher.RestoreCommand(e, presets)
		if err != nil {
			st.dbgf("restore process %s (%s): %v", e.Name, e.ID, err)
			continue
		}
		if e.AutoRestart {
			c.SetAutoRestart(true)
		}
	}

	var wg sync.WaitGroup

	// SIGWINCH. The kernel emits one signal per kernel-side resize, and
	// drag-resizes produce tens of them per second. The full
	// resize-redraw pipeline (ResizeAll + clearScreen + repaintFocused +
	// chrome) is expensive enough that running it per signal causes
	// visible scroll-jumping in diff-based TUIs like codex. Coalesce:
	// reset an ~80ms timer on every event, then run the pipeline once
	// when the timer fires. Skip repaintFocused on this path — the
	// child's own SIGWINCH-driven redraw fills the viewport; running
	// our snapshot replay over a child that's mid-reflow is what
	// produces the "crazy" scroll.
	wg.Add(1)
	winch := make(chan os.Signal, 1)
	signal.Notify(winch, syscall.SIGWINCH)
	go func() {
		defer wg.Done()
		defer signal.Stop(winch)
		const debounce = 80 * time.Millisecond
		var timer *time.Timer
		var timerC <-chan time.Time
		doResize := func() {
			c, r := hostSize()
			if c == 0 || r == 0 {
				return
			}
			st.dimsMu.Lock()
			st.hostCols, st.hostRows = c, r
			l := st.layoutLocked()
			st.dimsMu.Unlock()
			st.mu.Lock()
			if st.renderer != nil {
				st.renderer.SetLayout(l)
			}
			st.mu.Unlock()
			sess.ResizeAll(l.childCols(), l.childRows())
			launcher.SetSize(l.childCols(), l.childRows())
			host.SetSize(l.childCols(), l.childRows())
			st.clearScreen()
			st.drawTabBar()
			st.drawSidebar()
			st.drawStatusLine()
		}
		for {
			select {
			case <-ctx.Done():
				if timer != nil {
					timer.Stop()
				}
				return
			case <-winch:
				if timer == nil {
					timer = time.NewTimer(debounce)
					timerC = timer.C
				} else {
					if !timer.Stop() {
						select {
						case <-timer.C:
						default:
						}
					}
					timer.Reset(debounce)
				}
			case <-timerC:
				timer = nil
				timerC = nil
				doResize()
			}
		}
	}()

	// Chrome ticker: drain the dirty flag at ~60 Hz so per-chunk PTY
	// output doesn't pay tabbar/statusline rebuild cost on every chunk.
	wg.Add(1)
	go func() {
		defer wg.Done()
		ticker := time.NewTicker(16 * time.Millisecond)
		defer ticker.Stop()
		for {
			select {
			case <-ctx.Done():
				return
			case <-st.chromeWake:
			case <-ticker.C:
			}
			chromeChanged := st.chromeDirty.Swap(false)
			sidebarChanged := st.sidebarDirty.Swap(false)
			didWork := chromeChanged || sidebarChanged
			st.metrics.recordTickerFire(didWork)
			if !didWork {
				continue
			}
			if chromeChanged {
				st.drawTabBar()
				st.drawStatusLine()
			}
			if sidebarChanged {
				st.drawSidebar()
			}
		}
	}()

	// Marquee ticker: while a focused sidebar row's name overflows the
	// rail width, advance the pause-scroll-pause animation by marking
	// the sidebar dirty every marqueeStep. The chrome ticker above does
	// the actual repaint. When no row is animating, this is a single
	// cheap wakeup with no work.
	wg.Add(1)
	go func() {
		defer wg.Done()
		ticker := time.NewTicker(marqueeStep)
		defer ticker.Stop()
		for {
			select {
			case <-ctx.Done():
				return
			case <-ticker.C:
			}
			if st.marquee.active() {
				st.markSidebarDirty()
			}
		}
	}()

	// External termination: SPEC §2 step 4 (SIGTERM/SIGHUP → graceful exit).
	wg.Add(1)
	sigCh := make(chan os.Signal, 1)
	signal.Notify(sigCh, syscall.SIGTERM, syscall.SIGHUP)
	go func() {
		defer wg.Done()
		defer signal.Stop(sigCh)
		select {
		case <-ctx.Done():
			return
		case sig := <-sigCh:
			st.dbgf("signal %s; tearing down", sig)
			cancel()
		}
	}()

	// Stdin loop.
	go func() {
		if err := st.stdinLoop(); err != nil {
			st.dbgf("stdinLoop: %v", err)
		}
		cancel()
	}()

	<-ctx.Done()
	wg.Wait()
	st.leaveScreen()

	if restoreState != nil {
		_ = term.Restore(int(os.Stdin.Fd()), restoreState)
	}

	if st.lastExit.Load() >= 0 {
		fmt.Fprintf(os.Stderr, "patterm: last child exited (%d).\n", st.lastExit.Load())
	}
	return nil
}

// uiState is the shared state between the SIGWINCH loop, the stdin
// loop, and the session listener callbacks.
type uiState struct {
	sess     *Session
	presets  preset.Set
	launcher *Launcher
	pads     *scratchpad.Store
	trust    *trust.Store
	timers   *timerManager

	outMu sync.Mutex

	mu          sync.Mutex
	palette     *paletteState
	focusedID   string
	focusedName string
	// focusedPad names the scratchpad currently rendered in the main
	// viewport. When non-empty, focusedID is "" and the host renders
	// pad content instead of forwarding child PTY output. Mutually
	// exclusive with focusedID. The palette also reads this to surface
	// scratchpad-specific actions at the top of the command list.
	focusedPad string
	// padOffset is the index of the top-most rendered row in the
	// markdown-formatted view of focusedPad. Reset when focus moves to
	// a different pad; preserved across content changes for the same
	// pad so writes from MCP don't snap the user's view back to the
	// top.
	padOffset int
	// padOffsetName tracks which pad padOffset belongs to so a focus
	// switch resets the offset cleanly.
	padOffsetName string

	// activeAgentID tracks which top-level agent tab "owns" the agent
	// tree section of the sidebar. It only updates when focus lands on
	// an agent (or one of its sub-agents), so the agent tree stays
	// visible even when the user steps into the Processes pane.
	activeAgentID string
	// renderer confines focused-child live output to the main viewport.
	// A fresh renderer is allocated per focused child so partial-escape
	// state cannot bleed between panes.
	renderer             *viewportRenderer
	repaintNextPTY       string
	repaintNextPTYBudget int

	// toasts is the stackable notification surface. flashError,
	// flashTransient, and notifyAttention all push onto it; the user
	// dismisses entries with Ctrl-N or the "Clear notifications"
	// palette command.
	toasts toastStack

	// pendingTrust is the most recent trust prompt — surfaced in the
	// status line until the user resolves it with Ctrl-K. v1 keeps the
	// confirmation modal minimal: the user opens the palette and picks
	// "Trust preset <name>" / "Deny preset <name>". A future iteration
	// can promote this to a dedicated inline modal.
	pendingTrust *trustRequest

	dimsMu             sync.Mutex
	hostCols, hostRows uint16
	stdinTTY           bool

	// metrics is the optional performance tracker. nil when --profile
	// is off. Hot paths call metrics.recordX which is a fast nil
	// check on the disabled path.
	metrics *metricsTracker

	settingsMu   sync.Mutex
	settings     settings
	settingsPath string
	ctx          context.Context
	summaries    *summaryManager

	// chromeCacheMu guards the last-rendered byte cache for each chrome
	// element. The tab bar, sidebar, and status line all repaint on
	// many state changes and on every PTY chunk, but their content
	// usually doesn't change between calls — caching the rendered
	// output and skipping a write when it matches eliminates the
	// flicker (especially in the sidebar's session tree).
	chromeCacheMu   sync.Mutex
	tabBarCache     string
	sidebarCache    string
	statusLineCache string

	// chromeDirty defers tab-bar and status-line repaints off the
	// per-PTY-chunk hot path. OnPTYOut sets it; a ticker goroutine
	// drains it at ~60 Hz and runs the actual draw calls. Latency-
	// sensitive paths (owner flip, attention, trust, focus change)
	// continue to call drawStatusLine / drawTabBar synchronously.
	chromeDirty atomic.Bool
	// sidebarDirty defers sidebar repaints off the per-chunk hot path
	// in the same way. A long claude session resume — where every PTY
	// chunk scrolls the viewport — used to call drawSidebar()
	// synchronously per chunk, which dominated the resume's wall time
	// (hundreds of full-sidebar rebuilds for a frame that was almost
	// always cache-equal).
	sidebarDirty atomic.Bool
	chromeWake   chan struct{}

	// marquee animates the focused sidebar row's name when it overflows
	// the rail width. The dedicated 150ms ticker below flips
	// sidebarDirty while a row is animating; idle case is free.
	marquee marqueeState

	// padsCacheMu guards the cached scratchpad listing. The sidebar
	// and palette/sidebar nav helpers read it on every chunk-driven
	// repaint; the cache invalidates in scratchpadsChanged() which is
	// the canonical "pads mutated" signal from MCP write/append. nil
	// means "never read yet" — next caller refreshes.
	padsCacheMu sync.Mutex
	padsCache   []scratchpad.Entry

	lastExit atomic.Int32
}

func (st *uiState) dbgf(format string, args ...any) {
	logf(format, args...)
}

func (st *uiState) activeSummaryText(width int) string {
	text := st.activeSummaryRaw()
	if text == "" || width <= 0 {
		return ""
	}
	if visibleLen(text) > width {
		text = clipRunes(text, width-1) + "…"
	}
	return text
}

func (st *uiState) activeSummaryRaw() string {
	if st.summaries == nil {
		return ""
	}
	st.settingsMu.Lock()
	enabled := st.settings.AutoSummary.Enabled
	st.settingsMu.Unlock()
	if !enabled {
		return ""
	}
	st.mu.Lock()
	active := st.activeAgentID
	st.mu.Unlock()
	if active == "" {
		return ""
	}
	sum := st.summaries.Summary(active)
	text := strings.TrimSpace(sum.Text)
	if text == "" {
		return ""
	}
	return text
}

// trustRequest is one outstanding SPEC §7 trust prompt: an agent tried
// to spawn / start / restart against an untrusted command preset and
// the host wants user confirmation before the next attempt succeeds.
type trustRequest struct {
	processID  string
	presetName string
	reason     string
}

// promptTrust is the SPEC §7 trust gate UI hook. Replaces any prior
// pending request — the most recent prompt wins.
func (st *uiState) promptTrust(processID, presetName, reason string) {
	st.mu.Lock()
	st.pendingTrust = &trustRequest{processID: processID, presetName: presetName, reason: reason}
	st.mu.Unlock()
	st.drawStatusLine()
}

// focusProcess is the SPEC §7 select_process hook. Routes through the
// normal focus-change path; only takes effect if the process exists.
func (st *uiState) focusProcess(processID string) {
	c := st.sess.FindChild(processID)
	if c == nil {
		return
	}
	st.marquee.reset()
	layout := st.layoutSnapshot()
	onAlt := childIsOnAlt(c)
	st.mu.Lock()
	leavingPad := st.focusedPad != ""
	st.focusedPad = ""
	st.focusedID = c.ID
	st.focusedName = c.DisplayName()
	st.updateActiveAgentLocked(c)
	r := newViewportRenderer(layout)
	r.SetChildOnAlt(onAlt)
	st.renderer = r
	st.mu.Unlock()
	st.syncHostMouseForChild(onAlt)
	// Wipe whatever the previous focus (PTY child or pad view) left in
	// the viewport before painting the new child's snapshot.
	if leavingPad {
		st.clearViewportArea()
	}
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// childIsOnAlt reports whether the child's emulator is currently on
// its alternate screen. Returns false if the emulator is gone or the
// query fails.
func childIsOnAlt(c *Child) bool {
	if c == nil {
		return false
	}
	em := c.Emulator()
	if em == nil {
		return false
	}
	sc, err := em.ActiveScreen()
	if err != nil {
		return false
	}
	return sc == vt.ScreenAlternate
}

// syncHostMouseForChild emits the host mouse-reporting toggle that
// matches a newly-focused child's screen side. Primary-screen children
// want host mouse armed so the wheel drives inline scrollback; alt-
// screen children get host mouse disabled by default so click-and-drag
// selection works. Alt-screen TUIs that need mouse (vim, ranger, etc.)
// re-enable it themselves, and the viewport renderer forwards those
// toggles back to the host.
func (st *uiState) syncHostMouseForChild(onAlt bool) {
	st.outMu.Lock()
	defer st.outMu.Unlock()
	if onAlt {
		_, _ = os.Stdout.WriteString("\x1b[?1000l\x1b[?1006l")
	} else {
		_, _ = os.Stdout.WriteString("\x1b[?1000h\x1b[?1006h")
	}
}

// focusScratchpad shifts focus to a scratchpad. The main viewport
// renders the pad's text instead of any child PTY; PTY output for the
// previously focused child is dropped until focus moves back to a
// child. Empty name clears scratchpad focus.
func (st *uiState) focusScratchpad(name string) {
	if name == "" {
		return
	}
	st.marquee.reset()
	st.mu.Lock()
	if st.padOffsetName != name {
		st.padOffset = 0
		st.padOffsetName = name
	}
	st.focusedPad = name
	st.focusedID = ""
	st.focusedName = name
	st.renderer = nil
	st.mu.Unlock()
	st.clearViewportArea()
	st.repaintFocusedPad()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// clearViewportArea wipes the rectangle the focused-child PTY (or pad
// view) paints into so the next paint starts on a clean canvas. Used
// when transitioning between pad and child focus.
func (st *uiState) clearViewportArea() {
	layout := st.layoutSnapshot()
	mainBottom := int(layout.statusRow) - statusRows
	if mainBottom < int(layout.mainTop) {
		return
	}
	var b strings.Builder
	// ECH clears `mainCols` cells from each row in the viewport without
	// touching the sidebar columns.
	width := int(layout.childCols())
	for r := int(layout.mainTop); r <= mainBottom; r++ {
		fmt.Fprintf(&b, "\x1b[%d;%dH\x1b[%dX", r, int(layout.mainLeft), width)
	}
	st.outMu.Lock()
	defer st.outMu.Unlock()
	_, _ = os.Stdout.WriteString(b.String())
}

func (st *uiState) restartFocusedCommand(processID string) {
	c := st.sess.FindChild(processID)
	if c == nil || c.Kind != KindCommand {
		return
	}
	st.marquee.reset()
	layout := st.layoutSnapshot()
	renderer := newViewportRenderer(layout)
	st.mu.Lock()
	st.focusedID = c.ID
	st.focusedName = c.DisplayName()
	st.renderer = renderer
	st.repaintNextPTY = c.ID
	st.repaintNextPTYBudget = 2
	st.mu.Unlock()

	st.outMu.Lock()
	_, _ = os.Stdout.Write(renderer.ClearViewport())
	st.outMu.Unlock()

	if err := st.sess.Restart(c.ID, syscall.SIGTERM, layout.childCols(), layout.childRows()); err != nil {
		st.flashError(fmt.Sprintf("restart %s: %v", c.DisplayName(), err))
		return
	}
	st.moveToViewportOrigin()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// updateActiveAgentLocked records the active agent root for the agent
// tree section whenever focus lands on an agent or one of its
// sub-agents. Focusing a top-level command process leaves the previous
// active agent intact, so the user can hop between the Processes pane
// and the agent tree without losing context. Caller holds st.mu.
func (st *uiState) updateActiveAgentLocked(c *Child) {
	if c.Kind != KindAgent {
		return
	}
	if c.ParentID == "" {
		st.activeAgentID = c.ID
		return
	}
	// Walk up to the top-level agent.
	root := c
	for root.ParentID != "" {
		parent := st.sess.FindChild(root.ParentID)
		if parent == nil {
			break
		}
		root = parent
	}
	if root.Kind == KindAgent && root.ParentID == "" {
		st.activeAgentID = root.ID
	}
}

// notifyAttention is the request_human_attention sink (SPEC §7). We
// push a toast onto the stack; the focused-pane render path picks it
// up. The sidebar-blink is deferred until the §4 chrome lands.
func (st *uiState) notifyAttention(childID, reason string) {
	c := st.sess.FindChild(childID)
	name := childID
	if c != nil {
		name = c.DisplayName()
	}
	st.notifyToast(toastAttention, fmt.Sprintf("%s — %s", name, reason))
}

func (st *uiState) scratchpadsChanged() {
	st.padsCacheMu.Lock()
	st.padsCache = nil
	st.padsCacheMu.Unlock()
	st.chromeCacheMu.Lock()
	st.sidebarCache = ""
	st.chromeCacheMu.Unlock()
	st.drawSidebar()
	st.mu.Lock()
	focusedPad := st.focusedPad
	st.mu.Unlock()
	if focusedPad != "" {
		st.repaintFocusedPad()
	}
}

// OnChildSpawned auto-focuses the new child when the spawn came from
// the user (palette, persistence restore, or an external MCP client with
// no resolved identity). When ParentID is set — meaning a patterm-managed
// agent spawned this child via spawn_agent/spawn_process — focus stays
// on whatever the user was watching; the new child is still surfaced in
// the sidebar/tab bar so it's reachable via the palette or select_process.
func (st *uiState) OnChildSpawned(c *Child) {
	if st.summaries != nil {
		st.summaries.RegisterChild(c)
	}
	if c.ParentID != "" {
		st.mu.Lock()
		if st.palette != nil {
			st.palette.children = st.sess.Children()
			st.palette.focused = st.focusedID
			st.palette.rebuild()
			st.renderPaletteLocked()
		}
		st.mu.Unlock()
		st.drawTabBar()
		st.drawSidebar()
		return
	}
	st.marquee.reset()
	layout := st.layoutSnapshot()
	onAlt := childIsOnAlt(c)
	st.mu.Lock()
	st.focusedPad = ""
	st.focusedID = c.ID
	st.focusedName = c.DisplayName()
	st.updateActiveAgentLocked(c)
	renderer := newViewportRenderer(layout)
	renderer.SetChildOnAlt(onAlt)
	st.renderer = renderer
	palOpen := st.palette != nil
	if palOpen {
		st.palette.children = st.sess.Children()
		st.palette.focused = st.focusedID
		st.palette.rebuild()
		st.renderPaletteLocked()
	}
	// Prime the snapshot-replay budget for the new child. Diff-based
	// vendor TUIs (claude/codex/opencode) emit incremental updates that
	// assume the host display already matches their internal "last
	// frame" model. On a fresh spawn the host viewport was just cleared,
	// so incremental ops target cells that aren't populated yet —
	// leaving the corrupted pane the user works around by toggling
	// focus (which routes through repaintFocused). Setting the budget
	// here makes the next ~8 PTY chunks render from the full styled
	// emulator grid, so the host display tracks the emulator state
	// without needing a manual focus cycle.
	st.repaintNextPTY = c.ID
	st.repaintNextPTYBudget = 2
	st.mu.Unlock()

	// Wipe the viewport area so the previous focused child's PTY
	// output doesn't bleed through beneath the new pane. The palette
	// branch is skipped because the palette overlay covers the whole
	// screen and is about to take focus back to OnChildSpawned's
	// caller path.
	if !palOpen {
		st.outMu.Lock()
		_, _ = os.Stdout.Write(renderer.ClearViewport())
		st.outMu.Unlock()
	}

	st.syncHostMouseForChild(onAlt)
	st.moveToViewportOrigin()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// OnChildStateChanged repaints the sidebar and tab bar whenever a
// child's idle-state badge flips. Cheap — both draws bail when the
// cached frame hasn't changed.
func (st *uiState) OnChildStateChanged(string, IdleState) {
	st.drawTabBar()
	st.drawSidebar()
}

// OnChildClosed is the explicit-removal hook (close_process or the
// terminal-corpse cleanup in reapChild). The UI already reflects
// removals via the OnChildExited path and the children-map view, so
// this is a no-op here — the timerManager is the consumer that
// cares.
func (st *uiState) OnChildClosed(string) {}

// OnChildExited drops focus and shows the empty state if it was the
// focused child.
func (st *uiState) OnChildExited(c *Child) {
	if st.summaries != nil {
		st.summaries.UnregisterChild(c.ID)
	}
	st.lastExit.Store(int32(c.ExitCode()))
	st.marquee.reset()
	layout := st.layoutSnapshot()
	renderEmpty := false
	st.mu.Lock()
	if c.ID == st.focusedID {
		next := firstRunningTopLevel(st.sess.Children())
		if next == nil {
			st.focusedID = ""
			st.focusedName = ""
			renderEmpty = true
		} else {
			st.focusedID = next.ID
			st.focusedName = next.DisplayName()
			st.updateActiveAgentLocked(next)
			st.renderer = newViewportRenderer(layout)
		}
	}
	if c.ID == st.activeAgentID {
		// The active agent died; pin the agent tree to whatever agent
		// root is still running, or clear it if none remain.
		st.activeAgentID = firstRunningAgentID(st.sess.Children())
	}
	if st.palette != nil {
		st.palette.children = st.sess.Children()
		st.palette.focused = st.focusedID
		st.palette.rebuild()
		st.renderPaletteLocked()
	}
	repaint := st.focusedID != ""
	st.mu.Unlock()
	if renderEmpty {
		st.renderEmptyState()
	}
	if repaint {
		st.repaintFocused()
	}
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()

	// Auto-restart kicks in for command entries the user marked "relaunch
	// on exit". A short backoff (1s) avoids hot-spinning on processes
	// that fail immediately. The user can clear the flag by killing the
	// process from the palette.
	if c.Kind == KindCommand && c.AutoRestart() {
		go st.scheduleAutoRestart(c)
	}
}

// scheduleAutoRestart re-Starts a command entry after a brief backoff.
// Bails out if the user cleared the flag, closed the process, or the
// entry came back to life through some other path while we were
// waiting. Called as a goroutine from OnChildExited.
func (st *uiState) scheduleAutoRestart(c *Child) {
	time.Sleep(1 * time.Second)
	if !c.AutoRestart() {
		return
	}
	if st.sess.FindChild(c.ID) == nil {
		return
	}
	if c.IsLive() {
		return
	}
	l := st.layoutSnapshot()
	if err := st.sess.Start(c.ID, l.childCols(), l.childRows()); err != nil {
		st.dbgf("auto-restart %s: %v", c.ID, err)
		return
	}
	// Start doesn't fire emitSpawn, so we have to nudge the chrome
	// ourselves — the status flipped from exited back to running and
	// the sidebar's cached frame still shows the exited glyph.
	st.drawSidebar()
	st.drawStatusLine()
}

// OnPTYOut writes live output for the focused child when the palette is
// not covering the screen. The viewport renderer shifts cursor movement
// into the main pane and rewrites destructive clears. Host autowrap is
// disabled only around the replay so long styled runs cannot wrap into
// the right rail.
func (st *uiState) OnPTYOut(childID string, chunk []byte) {
	var entry time.Time
	if st.metrics != nil {
		entry = time.Now()
	}
	if st.summaries != nil {
		st.summaries.ObserveOutput(childID)
	}
	layout := st.layoutSnapshot()
	st.mu.Lock()
	focus := st.focusedID
	palOpen := st.palette != nil
	renderer := st.renderer
	forceRepaint := focus == childID && st.repaintNextPTY == childID && st.repaintNextPTYBudget > 0
	if forceRepaint {
		renderer = newViewportRenderer(layout)
		st.renderer = renderer
		st.repaintNextPTYBudget--
		if st.repaintNextPTYBudget == 0 {
			st.repaintNextPTY = ""
		}
	}
	st.mu.Unlock()
	if palOpen || focus != childID || renderer == nil {
		st.metrics.recordPTYOutDrop()
		return
	}
	var out []byte
	if forceRepaint {
		var snapStart time.Time
		if st.metrics != nil {
			snapStart = time.Now()
		}
		out = st.renderFocusedSnapshot(childID, renderer, layout)
		if st.metrics != nil {
			st.metrics.recordSnapshot(time.Since(snapStart))
		}
		if len(out) == 0 {
			return
		}
	} else {
		var rstart time.Time
		if st.metrics != nil {
			rstart = time.Now()
		}
		out = renderer.Render(chunk)
		if st.metrics != nil {
			st.metrics.recordRender(time.Since(rstart))
		}
	}
	// One write covers the autowrap-disable prelude, the chunk, the
	// autowrap-restore postlude, and (when a toast is up) the toast
	// overlay — four syscalls collapsed into one under outMu. The
	// sequences were already emitted atomically under the lock;
	// coalescing just halves the syscall count and makes claude's
	// continuous redraws + our toast layer land in the same frame so
	// the box doesn't flicker as the child paints over its cells.
	overlay := st.toastOverlayBytes()
	wrapped := make([]byte, 0, len(out)+len(overlay)+10)
	wrapped = append(wrapped, "\x1b[?7l"...)
	wrapped = append(wrapped, out...)
	wrapped = append(wrapped, "\x1b[?7h"...)
	wrapped = append(wrapped, overlay...)
	var wstart time.Time
	if st.metrics != nil {
		wstart = time.Now()
	}
	st.outMu.Lock()
	_, _ = os.Stdout.Write(wrapped)
	st.outMu.Unlock()
	if st.metrics != nil {
		st.metrics.recordStdout(time.Since(wstart), len(wrapped))
	}
	// RI / IND / NEL / SU / SD / IL / DL and bottom-margin LF / VT / FF
	// scroll content within the host's scroll region, which spans every
	// column — so any of them drags the right-hand sidebar's session-tree
	// entries along with the main pane. The viewport renderer flags any
	// chunk that scrolls; when set, drop the sidebar cache so the next
	// drawSidebar repaints over the clobber instead of hitting the cache
	// and leaving the gap visible.
	scrolled := renderer.TookScrollAction()
	if scrolled {
		st.chromeCacheMu.Lock()
		st.sidebarCache = ""
		st.chromeCacheMu.Unlock()
		// Defer the sidebar repaint to the chrome ticker. On a long
		// session resume every PTY chunk scrolls, and a synchronous
		// drawSidebar() per chunk dominates wall time even when the
		// frame ends up cache-equal — the rebuild work is unconditional.
		// The chrome ticker drains the dirty flag at ~60 Hz, so the
		// visible gap a scrolled chunk can leave in the sidebar columns
		// is bounded by one frame.
		st.markSidebarDirty()
	}
	// Defer the tab bar + status line repaint to the chrome ticker.
	// The cached frame already short-circuits the wire write, but
	// avoiding the string build, FindChild, and locking on every
	// chunk pulls steady-state CPU off the hot path.
	st.markChromeDirty()
	if st.metrics != nil {
		st.metrics.recordPTYOut(time.Since(entry), len(chunk))
	}
}

func (st *uiState) enterScreen() {
	st.outMu.Lock()
	// SGR mouse reporting (?1000h ?1006h) stays on the entire time patterm
	// is on the alt screen so we always receive wheel events. The focused
	// child's wheel handling in processStdin decides whether each event
	// scrolls the viewport (primary screen) or forwards to the child
	// (alt screen / pad / palette).
	_, _ = os.Stdout.Write([]byte("\x1b[?1049h\x1b[H\x1b[2J\x1b[?25h\x1b[?1000h\x1b[?1006h"))
	st.outMu.Unlock()
	st.installHostScrollRegion()
}

func (st *uiState) leaveScreen() {
	st.outMu.Lock()
	defer st.outMu.Unlock()
	// Tear down any mouse reporting patterm enabled before leaving the
	// alt screen; otherwise the calling shell can be left with a host
	// that still emits SGR mouse events. Reset DECSTBM so the calling
	// shell isn't stuck with a constrained scroll region.
	_, _ = os.Stdout.Write([]byte("\x1b[r\x1b[?6l\x1b[?1006l\x1b[?1000l\x1b[?25h\x1b[?1049l"))
}

func (st *uiState) clearScreen() {
	st.invalidateChromeCache()
	st.outMu.Lock()
	_, _ = os.Stdout.Write([]byte("\x1b[?25h\x1b[H\x1b[2J"))
	st.outMu.Unlock()
	// Re-arm the host scroll region so the post-clear paint inherits
	// the viewport bounds. Without this, a SIGWINCH-driven clearScreen
	// followed by a long burst of child output (no DECSTBM of its own)
	// would scroll the host's full screen — chrome included — every
	// time the cursor reached the bottom row.
	st.installHostScrollRegion()
}

// installHostScrollRegion writes DECSTBM to bound the host's scroll
// region to mainTop..mainBottom, then disables origin mode and CUPs
// back to viewport-top. With this in place a child that emits LF / IND
// / NEL / RI / SU / SD / IL / DL at the bottom of the viewport scrolls
// only within the viewport rows — the tab bar and status row never see
// the scroll. renderFocusedSnapshot already emits the same prelude for
// snapshot replays; this method covers the windows in between (initial
// startup, post-SIGWINCH, post-clearScreen) when no snapshot fires.
func (st *uiState) installHostScrollRegion() {
	layout := st.layoutSnapshot()
	mainBottom := int(layout.statusRow) - statusRows
	if mainBottom < int(layout.mainTop) {
		return
	}
	st.outMu.Lock()
	defer st.outMu.Unlock()
	fmt.Fprintf(os.Stdout, "\x1b[?6l\x1b[%d;%dr\x1b[%d;%dH",
		int(layout.mainTop), mainBottom,
		int(layout.mainTop), int(layout.mainLeft))
}

// invalidateChromeCache forces the next drawTabBar / drawSidebar /
// drawStatusLine call to actually emit bytes, regardless of cached
// content. Anything that clears or repaints the screen (resize, focus
// change, full repaint) must call this — otherwise the chrome stays
// blank because the cached frame still matches the unchanged state
// even though the wire was cleared.
// padsList returns the cached scratchpad listing, refreshing from
// disk on the first call after invalidation. Callers must not mutate
// the returned slice — it is shared.
func (st *uiState) padsList() []scratchpad.Entry {
	st.padsCacheMu.Lock()
	if st.padsCache != nil {
		out := st.padsCache
		st.padsCacheMu.Unlock()
		return out
	}
	st.padsCacheMu.Unlock()
	entries, err := st.pads.List()
	if err != nil {
		return nil
	}
	st.padsCacheMu.Lock()
	st.padsCache = entries
	st.padsCacheMu.Unlock()
	return entries
}

// markChromeDirty schedules a chrome (tab bar + status line) repaint
// on the next ticker frame. Cheap to call from the per-PTY-chunk hot
// path. Latency-sensitive sites (focus change, owner flip, attention,
// trust prompts) keep calling drawTabBar / drawStatusLine directly.
func (st *uiState) markChromeDirty() {
	st.chromeDirty.Store(true)
	select {
	case st.chromeWake <- struct{}{}:
	default:
	}
}

// markSidebarDirty schedules a sidebar repaint on the next ticker
// frame. Hot path — every scrolled PTY chunk lands here. Synchronous
// repaints from latency-sensitive sites (spawn, exit, focus, state
// change, trust) keep calling drawSidebar directly.
func (st *uiState) markSidebarDirty() {
	st.sidebarDirty.Store(true)
	select {
	case st.chromeWake <- struct{}{}:
	default:
	}
}

func (st *uiState) invalidateChromeCache() {
	st.chromeCacheMu.Lock()
	st.tabBarCache = ""
	st.sidebarCache = ""
	st.statusLineCache = ""
	st.chromeCacheMu.Unlock()
}

func (st *uiState) moveToViewportOrigin() {
	layout := st.layoutSnapshot()
	st.outMu.Lock()
	defer st.outMu.Unlock()
	fmt.Fprintf(os.Stdout, "\x1b[%d;%dH", int(layout.mainTop), int(layout.mainLeft))
}

func (st *uiState) renderPaletteLocked() {
	if st.palette == nil {
		return
	}
	st.outMu.Lock()
	defer st.outMu.Unlock()
	cols, rows := st.hostSizeSnapshot()
	st.palette.render(wrapWriter(os.Stdout), int(cols), int(rows))
}

// drawStatusLine renders SPEC §4's bottom status line. Left side: input
// ownership toast ("orchestrator driving" / "you have control") and any
// attention ask. Right side: palette hint. The PTY child occupies
// host_rows-1 rows so this row is exclusively ours.
func (st *uiState) drawStatusLine() {
	var entry time.Time
	if st.metrics != nil {
		entry = time.Now()
	}
	st.mu.Lock()
	palOpen := st.palette != nil
	focusID := st.focusedID
	focusName := st.focusedName
	var trustMsg string
	if st.pendingTrust != nil {
		trustMsg = fmt.Sprintf("trust preset %q? [y]es / [n]o", st.pendingTrust.presetName)
	}
	st.mu.Unlock()
	if palOpen {
		return
	}
	cols, rows := st.hostSizeSnapshot()
	if cols == 0 || rows == 0 {
		return
	}
	// Resolve the focused child once — drawStatusLine fires on every
	// PTY chunk and ticker tick, and FindChild takes the session
	// mutex.
	var focusedChild *Child
	if focusID != "" {
		focusedChild = st.sess.FindChild(focusID)
	}
	owner := ""
	if focusedChild != nil {
		switch focusedChild.Owner() {
		case OwnerOrchestrator:
			owner = "orchestrator driving"
		case OwnerUser:
			owner = "you have control"
		}
	}
	left := ""
	if focusName != "" {
		left = focusName
	}
	if owner != "" {
		if left != "" {
			left = left + " · " + owner
		} else {
			left = owner
		}
	}
	if trustMsg != "" {
		left = "[trust] " + trustMsg
	}
	// Hints decay left-to-right when the host is narrow so the focused
	// child name + ownership note on the left side never get clipped.
	// Context-specific hints are appended so they survive longest.
	hints := []string{
		"Ctrl-A/D · tabs",
		"Ctrl-W/S · tree",
		"Ctrl-K · palette",
	}
	if focusedChild != nil {
		hints = append(hints, "Ctrl-B · scroll")
		if focusedChild.Kind == KindCommand {
			hints = append(hints, "Ctrl-R · restart")
		}
	}
	// Surface the toast-dismiss chord only while a notification is on
	// screen — the hint is noise otherwise, and Ctrl-N falls through
	// to the focused PTY when the stack is empty.
	if st.toasts.length() > 0 {
		hints = append(hints, "Ctrl-N · dismiss")
	}
	right := strings.Join(hints, "  ·  ")
	for len(hints) > 1 && int(cols)-len(left)-len(right) < 1 {
		hints = hints[1:]
		right = strings.Join(hints, "  ·  ")
	}

	pad := int(cols) - len(left) - len(right)
	if pad < 1 {
		pad = 1
	}
	line := left + strings.Repeat(" ", pad) + right
	if len(line) > int(cols) {
		line = line[:int(cols)]
	}
	st.chromeCacheMu.Lock()
	if line == st.statusLineCache {
		st.chromeCacheMu.Unlock()
		if st.metrics != nil {
			st.metrics.recordStatus(time.Since(entry), true)
		}
		return
	}
	st.statusLineCache = line
	st.chromeCacheMu.Unlock()
	if st.metrics != nil {
		defer func() { st.metrics.recordStatus(time.Since(entry), false) }()
	}

	st.outMu.Lock()
	defer st.outMu.Unlock()
	// Save cursor, move to last row col 1, write, restore.
	fmt.Fprintf(os.Stdout, "\x1b7\x1b[999;1H\x1b[2m\x1b[7m%s\x1b[0m\x1b8", line)
}

// renderEmptyState is the SPEC §4 blank-canvas hint. Drawn whenever no
// child is focused.
func (st *uiState) renderEmptyState() {
	layout := st.layoutSnapshot()
	line := "Press Ctrl-K to spawn an agent or process"
	row := int(layout.mainTop) + (int(layout.childRows()) / 2)
	col := int(layout.mainLeft) + ((int(layout.childCols()) - len(line)) / 2)
	if row < int(layout.mainTop) {
		row = int(layout.mainTop)
	}
	if col < int(layout.mainLeft) {
		col = int(layout.mainLeft)
	}
	st.outMu.Lock()
	fmt.Fprintf(os.Stdout, "\x1b[?25l\x1b[H\x1b[2J\x1b[%d;%dH\x1b[2m%s\x1b[0m", row, col, line)
	st.outMu.Unlock()
	st.renderToasts()
}

func (st *uiState) hostSizeSnapshot() (uint16, uint16) {
	st.dimsMu.Lock()
	defer st.dimsMu.Unlock()
	return st.hostCols, st.hostRows
}

func (st *uiState) layoutSnapshot() terminalLayout {
	st.dimsMu.Lock()
	defer st.dimsMu.Unlock()
	return st.layoutLocked()
}

func (st *uiState) layoutLocked() terminalLayout {
	return newTerminalLayout(st.hostCols, st.hostRows)
}

// splitOnEnter walks input and returns each Enter byte (CR or LF) as
// its own slice, with the surrounding non-Enter bytes batched between.
// Empty pieces are dropped. The result preserves byte order, so
// "hello\rworld\n" yields ["hello", "\r", "world", "\n"]. Callers use
// this to keep Enter keystrokes from getting bundled into the same
// PTY write as the text that preceded them — TUI agents' paste
// detection (claude/codex/opencode) otherwise swallows the CR as
// literal content instead of treating it as a key event.
func splitOnEnter(in []byte) [][]byte {
	if len(in) == 0 {
		return nil
	}
	var out [][]byte
	start := 0
	for i, b := range in {
		if b != '\r' && b != '\n' {
			continue
		}
		if i > start {
			out = append(out, in[start:i])
		}
		out = append(out, in[i:i+1])
		start = i + 1
	}
	if start < len(in) {
		out = append(out, in[start:])
	}
	return out
}

func (st *uiState) stdinLoop() error {
	buf := make([]byte, 4096)
	for {
		n, err := os.Stdin.Read(buf)
		if n > 0 {
			st.processStdin(buf[:n])
		}
		if err != nil {
			if errors.Is(err, io.EOF) {
				return nil
			}
			return fmt.Errorf("read error %w (n=%d)", err, n)
		}
	}
}

// processStdin walks one read of stdin byte by byte. The palette
// intercepts everything when it's open. Otherwise Ctrl-K opens it and
// every other byte forwards to the focused PTY. The Ctrl-K Ctrl-K chord
// is SPEC §4's passthrough prefix: after the first Ctrl-K, if the very
// next byte is another Ctrl-K, both are sent to the PTY literally.
//
// NOTE on locking: a palette-close action (spawn / switch / kill /
// quit) may fire session listeners (OnChildSpawned, OnChildExited)
// synchronously. Those listeners need st.mu. We must NOT hold st.mu
// when calling closePalette — bytes after the action in the same chunk
// are dropped on the floor, which is the right behavior anyway (the
// user just decided the prior pane is gone).
func (st *uiState) processStdin(chunk []byte) {
	st.mu.Lock()

	// Trust modal is modal: y/Y accepts, n/N or ESC denies. Everything
	// else is ignored so a typo doesn't leak into the focused PTY while
	// the prompt is up. SPEC §7 trust gate.
	if st.pendingTrust != nil {
		req := *st.pendingTrust
		consumed := 0
		var resolved string
		for _, b := range chunk {
			consumed++
			switch b {
			case 'y', 'Y':
				resolved = "accept"
			case 'n', 'N', 0x1b: // ESC
				resolved = "deny"
			default:
				continue
			}
			break
		}
		if resolved != "" {
			st.pendingTrust = nil
			st.mu.Unlock()
			if resolved == "accept" {
				if err := st.trust.Grant(req.presetName); err != nil {
					st.flashError(fmt.Sprintf("trust grant: %v", err))
				} else {
					st.flashTransient(fmt.Sprintf("trusted preset %q (retry the call)", req.presetName))
				}
			} else {
				st.flashTransient(fmt.Sprintf("denied trust for preset %q", req.presetName))
			}
			st.drawStatusLine()
			// Discard the rest of the chunk; we intentionally don't
			// recurse into the regular handler so a stray Enter doesn't
			// submit anything to the focused PTY.
			_ = consumed
			return
		}
		st.mu.Unlock()
		return
	}

	forward := make([]byte, 0, len(chunk))

	var pendingAction *paletteAction
	var pendingNav navEntry
	var pendingRestartID string
	var pendingViewportDelta int
	var pendingViewportBottom bool
	var pendingPadStep int
	var pendingPadExit bool
	var pendingDismissToast bool

	flushForward := func() {
		if len(forward) == 0 {
			return
		}
		if st.focusedID != "" {
			if c := st.sess.FindChild(st.focusedID); c != nil && c.Status() == StatusRunning {
				prev := c.Owner()
				// InjectAsUser splits Enter bytes onto their own
				// writes so claude / codex / opencode don't treat a
				// "text\r" batch as a paste.
				_ = c.InjectAsUser(forward)
				if st.summaries != nil {
					st.summaries.ObserveHumanInput(c.ID, forward)
				}
				if prev != OwnerUser {
					go st.drawStatusLine()
				}
				// Auto-snap the emulator viewport to the live area
				// on any forwarded keystroke. Without this, typing
				// while scrolled into history leaves the cursor /
				// echoed bytes off-screen below the visible region.
				pendingViewportBottom = true
			}
		}
		forward = forward[:0]
	}

	// childOnPrimary captures whether the focused child is on its primary
	// screen at the start of this chunk. Wheel events on the primary
	// screen scroll the emulator viewport (inline scrollback); on the
	// alternate screen they fall through to the child PTY so vim / less /
	// codex can consume them.
	childOnPrimary := false
	if st.focusedID != "" {
		if c := st.sess.FindChild(st.focusedID); c != nil {
			if em := c.Emulator(); em != nil {
				if sc, err := em.ActiveScreen(); err == nil && sc == vt.ScreenPrimary {
					childOnPrimary = true
				}
			}
		}
	}

	// Tracks the last arrow direction and the byte offset immediately
	// after its CSI sequence. Some terminals emit a duplicate adjacent
	// arrow event for one physical keypress (legacy `CSI B` + kitty
	// `CSI 57353 u`, or two of the same form back-to-back). We collapse
	// those into a single navigation step. Any non-arrow byte resets the
	// tracker so genuine consecutive presses across other input still
	// register normally.
	var lastNav byte
	var lastNavEnd int

	i := 0
	for i < len(chunk) {
		b := chunk[i]

		// Scratchpad mode: pad has no PTY destination, so input is
		// repurposed for scrolling the rendered markdown view.
		// Scroll-wheel events are the primary control (we enable SGR
		// mouse reporting in focusScratchpad); arrow keys / PgUp/PgDn /
		// Home / End work for keyboard users. App-level chords (Ctrl-K
		// palette, Ctrl-WASD focus, Ctrl-B scrollback) fall through to
		// the handlers below; everything else is swallowed silently so
		// typing into a pad view can't leak to a child PTY.
		//
		// When the palette is open we skip this block entirely so the
		// palette handler below receives every byte. Otherwise typing
		// (and Esc) get swallowed here and the palette appears wedged.
		if st.focusedPad != "" && st.palette == nil {
			if b == 0x1b { // ESC or CSI
				if n := csiLen(chunk, i); n > 0 {
					final := chunk[i+n-1]
					params := chunk[i+2 : i+n-1]
					// SGR mouse: `CSI < button ; col ; row M/m`. We
					// enabled 1006 reporting on focus, so the host emits
					// this form. Wheel-up = 64, wheel-down = 65; +shift
					// adds 4 → 68/69; +ctrl adds 16 → 80/81. We treat
					// any wheel button as a 3-row step.
					if final == 'M' && len(params) > 0 && params[0] == '<' {
						if step, ok := parseSGRMouseWheel(params[1:]); ok {
							pendingPadStep += step
							i += n
							continue
						}
						// Non-wheel mouse event (click/drag/release):
						// drop silently. Pads don't have a click model
						// yet, and forwarding to a child would be
						// confusing while the pad view is up.
						i += n
						continue
					}
					if final == 'm' && len(params) > 0 && params[0] == '<' {
						// SGR release event — always drop.
						i += n
						continue
					}
					switch final {
					case 'A':
						pendingPadStep -= 1
						i += n
						continue
					case 'B':
						pendingPadStep += 1
						i += n
						continue
					case '~':
						pstr := string(params)
						layout := st.layoutLocked()
						page := int(layout.childRows()) - 2
						if page < 1 {
							page = 1
						}
						switch pstr {
						case "5":
							pendingPadStep -= page
							i += n
							continue
						case "6":
							pendingPadStep += page
							i += n
							continue
						case "1", "7":
							pendingPadStep -= 1 << 30
							i += n
							continue
						case "4", "8":
							pendingPadStep += 1 << 30
							i += n
							continue
						}
					case 'u':
						if k, ok := decodeCSIu(string(params)); ok && k.event == 1 {
							switch k.key {
							case kittyKeyUp:
								pendingPadStep -= 1
								i += n
								continue
							case kittyKeyDown:
								pendingPadStep += 1
								i += n
								continue
							}
						}
					}
					// Unhandled CSI: drop so the pad view stays stable
					// instead of letting stray escapes hit the next
					// handler block.
					i += n
					continue
				}
				// Legacy X10 mouse: `CSI M Cb Cx Cy`, three raw bytes
				// after the M. csiLen consumed only up to 'M'; pick up
				// the three trailing bytes here. Cb is button + 32;
				// wheel = 64 → byte 96, wheel-down = 65 → byte 97.
				if i+5 < len(chunk) && chunk[i+1] == '[' && chunk[i+2] == 'M' {
					cb := chunk[i+3]
					switch cb {
					case 96, 100, 112: // 64, 68, 80 — wheel up variants
						pendingPadStep -= 3
						i += 6
						continue
					case 97, 101, 113: // 65, 69, 81 — wheel down variants
						pendingPadStep += 3
						i += 6
						continue
					}
					// Non-wheel legacy mouse: drop the 6-byte event.
					i += 6
					continue
				}
				// Bare ESC exits the pad view.
				pendingPadExit = true
				i++
				break
			}
			// Plain bytes (letters, control chars other than ESC) drop
			// silently except for the app-level chords we explicitly
			// allow through below.
			if hit, _ := matchCtrlK(chunk, i); hit {
				// fall through to the app-level handler
			} else if hit, _ := matchCtrlChar(chunk, i, 'a'); hit {
			} else if hit, _ := matchCtrlChar(chunk, i, 'd'); hit {
			} else if hit, _ := matchCtrlChar(chunk, i, 'w'); hit {
			} else if hit, _ := matchCtrlChar(chunk, i, 's'); hit {
			} else if hit, _ := matchCtrlChar(chunk, i, 'n'); hit {
				// Ctrl-N is the toast dismiss key. In pad view we
				// allow it through the chord block so the handler
				// below can fire even though pads otherwise swallow
				// bytes.
			} else {
				i++
				continue
			}
		}

		// Palette mode swallows all bytes.
		if st.palette != nil {
			if nav, navLen := peekArrowEvent(chunk, i); nav != 0 {
				if i == lastNavEnd && nav == lastNav {
					i += navLen
					continue
				}
				lastNav = nav
				lastNavEnd = i + navLen
			} else {
				lastNav = 0
				lastNavEnd = -1
			}

			action, done, adv := st.palette.handleInput(chunk, i)
			if adv <= 0 {
				adv = 1
			}
			i += adv
			if action.kind == "settings-save" {
				st.applySettingsAction(action)
				st.renderPaletteLocked()
				continue
			}
			if done {
				a := action
				pendingAction = &a
				break
			}
			st.renderPaletteLocked()
			continue
		}

		// Ctrl-K is the reserved app-level binding. Two cases:
		//   - Ctrl-K then anything except Ctrl-K → open palette.
		//   - Ctrl-K Ctrl-K → forward both keystrokes to the child raw.
		//
		// Ctrl-K is recognised in legacy (0x0B), kitty CSI u, and xterm
		// modifyOtherKeys encodings — see matchCtrlK. The chord forwards
		// the bytes the terminal actually emitted, so a child that asked
		// for kitty input gets kitty input.
		if hit, adv := matchCtrlK(chunk, i); hit {
			if hit2, adv2 := matchCtrlK(chunk, i+adv); hit2 {
				flushForward()
				forward = append(forward, chunk[i:i+adv+adv2]...)
				flushForward()
				i += adv + adv2
				continue
			}
			flushForward()
			st.openPaletteLocked()
			i += adv
			continue
		}

		// Ctrl+WASD: directional focus navigation, matching the four
		// arrow keys you'd expect in a tiling layout. A/D step between
		// top-level tabs; W/S step through the current tab's process
		// list (root first, then sub-agents). Bytes after the chord
		// in the same chunk are dropped — the focus change makes
		// further forwarding ambiguous between old and new pane.
		if hit, adv := matchCtrlChar(chunk, i, 'a'); hit {
			flushForward()
			if id := nextTabID(st.sess.Children(), st.focusedID, -1); id != "" {
				pendingNav = navEntry{childID: id}
			}
			i += adv
			break
		}
		if hit, adv := matchCtrlChar(chunk, i, 'd'); hit {
			flushForward()
			if id := nextTabID(st.sess.Children(), st.focusedID, +1); id != "" {
				pendingNav = navEntry{childID: id}
			}
			i += adv
			break
		}
		if hit, adv := matchCtrlChar(chunk, i, 'w'); hit {
			flushForward()
			pendingNav = nextNavEntry(st.sess.Children(), st.focusedID, st.focusedPad, st.activeAgentID, st.padsList(), -1)
			i += adv
			break
		}
		if hit, adv := matchCtrlChar(chunk, i, 's'); hit {
			flushForward()
			pendingNav = nextNavEntry(st.sess.Children(), st.focusedID, st.focusedPad, st.activeAgentID, st.padsList(), +1)
			i += adv
			break
		}
		if hit, adv := matchCtrlChar(chunk, i, 'r'); hit {
			if c := st.sess.FindChild(st.focusedID); c != nil && c.Kind == KindCommand {
				flushForward()
				pendingRestartID = c.ID
				i += adv
				break
			}
		}
		// Ctrl-N dismisses the most recent toast. We only consume the
		// chord when there's actually a toast to dismiss; otherwise the
		// bytes fall through to the focused PTY so readline /
		// nano / emacs / opencode keep working in shells and editors.
		if hit, adv := matchCtrlChar(chunk, i, 'n'); hit {
			if st.toasts.length() > 0 {
				flushForward()
				pendingDismissToast = true
				i += adv
				continue
			}
			forward = append(forward, chunk[i:i+adv]...)
			i += adv
			continue
		}

		// Ctrl-B snaps the focused child's emulator viewport back to the
		// active area. Use this as the escape hatch from a scrolled-up
		// state — wheel scrolls move the viewport into the libghostty
		// scrollback history; Ctrl-B brings it back. The chord is
		// intercepted before forwarding so the child shell doesn't see a
		// stray Ctrl-B (readline backward-char).
		if hit, adv := matchCtrlChar(chunk, i, 'b'); hit {
			if st.focusedID != "" {
				flushForward()
				pendingViewportBottom = true
				i += adv
				continue
			}
		}

		// Inline wheel scrollback for a focused child on the primary
		// screen. The host always has SGR mouse reporting armed (see
		// enterScreen), so wheel events arrive here even when the child
		// shell never asked for mouse input. On the alternate screen we
		// let the bytes fall through to forward so vim / less / codex
		// receive the wheel event as input.
		if childOnPrimary && b == 0x1b {
			if n := csiLen(chunk, i); n > 0 {
				final := chunk[i+n-1]
				params := chunk[i+2 : i+n-1]
				if final == 'M' && len(params) > 0 && params[0] == '<' {
					if step, ok := parseSGRMouseWheel(params[1:]); ok {
						pendingViewportDelta += step
						i += n
						continue
					}
				}
			}
			// Legacy X10 mouse wheel: `CSI M Cb Cx Cy`.
			if i+5 < len(chunk) && chunk[i+1] == '[' && chunk[i+2] == 'M' {
				cb := chunk[i+3]
				switch cb {
				case 96, 100, 112:
					pendingViewportDelta -= 3
					i += 6
					continue
				case 97, 101, 113:
					pendingViewportDelta += 3
					i += 6
					continue
				}
			}
		}

		forward = append(forward, b)
		i++
	}
	flushForward()
	st.mu.Unlock()

	if pendingAction != nil {
		st.closePalette(*pendingAction)
	}
	if !pendingNav.empty() {
		switch {
		case pendingNav.isPad():
			st.focusScratchpad(pendingNav.pad)
		case pendingNav.isChild():
			st.focusProcess(pendingNav.childID)
		}
	}
	if pendingRestartID != "" {
		st.restartFocusedCommand(pendingRestartID)
	}
	if pendingViewportDelta != 0 {
		st.scrollFocusedViewport(pendingViewportDelta)
	}
	if pendingViewportBottom {
		st.scrollFocusedViewportToBottom()
	}
	if pendingPadStep != 0 {
		st.padScroll(pendingPadStep)
	}
	if pendingPadExit {
		st.exitPadView()
	}
	if pendingDismissToast {
		if st.toasts.dismissTop() {
			st.refreshToastSurface()
		}
	}
}

// scrollFocusedViewport scrolls the focused child's emulator viewport by
// `delta` rows (negative is up into scrollback history, positive is down
// towards the active area) and repaints the main pane against the new
// snapshot. No-op if no child is focused or the emulator isn't live yet.
func (st *uiState) scrollFocusedViewport(delta int) {
	st.mu.Lock()
	id := st.focusedID
	st.mu.Unlock()
	if id == "" {
		return
	}
	c := st.sess.FindChild(id)
	if c == nil {
		return
	}
	em := c.Emulator()
	if em == nil {
		return
	}
	if err := em.ScrollViewportDelta(delta); err != nil {
		return
	}
	st.repaintFocused()
}

// scrollFocusedViewportToBottom snaps the focused child's emulator
// viewport back to the active (live) area. Bound to Ctrl-B as the escape
// hatch from a scrolled-up state.
func (st *uiState) scrollFocusedViewportToBottom() {
	st.mu.Lock()
	id := st.focusedID
	st.mu.Unlock()
	if id == "" {
		return
	}
	c := st.sess.FindChild(id)
	if c == nil {
		return
	}
	em := c.Emulator()
	if em == nil {
		return
	}
	if err := em.ScrollViewportBottom(); err != nil {
		return
	}
	st.repaintFocused()
}

func (st *uiState) openPaletteLocked() {
	st.settingsMu.Lock()
	appSettings := st.settings.clone()
	st.settingsMu.Unlock()
	st.palette = newPalette(st.sess.Children(), st.focusedID, st.focusedPad, st.presets, appSettings)
	// Push a "no kitty flags" entry onto the host terminal's keyboard
	// stack so palette input arrives in plain legacy form regardless of
	// what the focused child pushed. Codex/ratatui enables kitty mode
	// for its own PTY; that push gets forwarded to the host and leaves
	// the host emitting arrow keys in multiple forms, which manifests
	// as the palette double-stepping on Down/Up. Popped on close.
	st.outMu.Lock()
	_, _ = os.Stdout.WriteString("\x1b[>0u")
	st.outMu.Unlock()
	st.renderPaletteLocked()
}

// closePalette is invoked with st.mu UNLOCKED. The session-mutating
// actions below (spawn / kill) fire listeners that take st.mu, so
// holding it here would deadlock. Each helper this calls takes its own
// brief mu acquisitions as needed.
func (st *uiState) closePalette(action paletteAction) {
	st.mu.Lock()
	st.palette = nil
	st.mu.Unlock()
	// Pair with the push in openPaletteLocked: restore whatever
	// keyboard flags the focused child had configured.
	st.outMu.Lock()
	_, _ = os.Stdout.WriteString("\x1b[<u")
	st.outMu.Unlock()
	st.clearScreen()

	// When a scratchpad is focused, the "main viewport" is showing the
	// pad's rendered body, not a child PTY. repaintFocused() would draw
	// an empty state (focusedID == "") and leave the previous palette's
	// top border visible in the pad area. Use the pad-aware helper for
	// any branch below that wants to restore the prior view.
	restoreView := func() {
		st.mu.Lock()
		padFocused := st.focusedPad != ""
		st.mu.Unlock()
		if padFocused {
			st.repaintFocusedPad()
			return
		}
		st.repaintFocused()
	}

	switch action.kind {
	case "", "cancel":
		restoreView()
		st.drawTabBar()
		st.drawSidebar()
		st.drawStatusLine()

	case "spawn-agent":
		if action.preset == nil {
			restoreView()
			return
		}
		l := st.layoutSnapshot()
		st.launcher.SetSize(l.childCols(), l.childRows())
		// LaunchAgent fires OnChildSpawned synchronously; it will draw
		// chrome and set focus.
		if _, err := st.launcher.LaunchAgent(action.preset, action.preset.Name, "", ""); err != nil {
			st.flashError(fmt.Sprintf("spawn %s: %v", action.preset.Name, err))
		}

	case "spawn-process":
		if action.preset == nil {
			restoreView()
			return
		}
		l := st.layoutSnapshot()
		st.launcher.SetSize(l.childCols(), l.childRows())
		if _, err := st.launcher.LaunchCommandPreset(action.preset, action.preset.Name, ""); err != nil {
			st.flashError(fmt.Sprintf("spawn %s: %v", action.preset.Name, err))
		}

	case "spawn-terminal":
		l := st.layoutSnapshot()
		st.launcher.SetSize(l.childCols(), l.childRows())
		if _, err := st.launcher.LaunchTerminal(nil, "terminal", "", "", nil); err != nil {
			st.flashError(fmt.Sprintf("spawn terminal: %v", err))
		}

	case "spawn-process-submit":
		if action.command == "" {
			restoreView()
			return
		}
		l := st.layoutSnapshot()
		st.launcher.SetSize(l.childCols(), l.childRows())
		display := action.command
		if len(display) > 32 {
			display = display[:31] + "…"
		}
		// shell=true so multi-word commands like "bun run dev" pass
		// through `sh -lc` and the user's PATH resolves binaries the
		// way they expect from an interactive shell.
		c, err := st.launcher.LaunchCommandArgv([]string{action.command}, display, "", "", nil, true)
		if err != nil {
			st.flashError(fmt.Sprintf("spawn: %v", err))
			return
		}
		c.SetAutoRestart(action.relaunch)
		// LaunchCommandArgv fires OnChildSpawned synchronously, which
		// drew the sidebar before AutoRestart was set. Invalidate so the
		// ⟳ marker shows up on the next paint.
		if action.relaunch {
			st.chromeCacheMu.Lock()
			st.sidebarCache = ""
			st.chromeCacheMu.Unlock()
			st.drawSidebar()
		}

	case "switch":
		c := st.sess.FindChild(action.childID)
		if c == nil || (c.Kind == KindAgent && c.Status() != StatusRunning) {
			restoreView()
			return
		}
		layout := st.layoutSnapshot()
		st.mu.Lock()
		leavingPad := st.focusedPad != ""
		st.focusedPad = ""
		st.focusedID = action.childID
		st.focusedName = c.DisplayName()
		st.updateActiveAgentLocked(c)
		st.renderer = newViewportRenderer(layout)
		st.mu.Unlock()
		// Switching from a pad to a child: wipe the pad body so the
		// child's snapshot paints onto a clean canvas, mirroring
		// focusProcess.
		if leavingPad {
			st.clearViewportArea()
		}
		st.repaintFocused()
		st.drawTabBar()
		st.drawSidebar()
		st.drawStatusLine()

	case "kill":
		// User-initiated kill cancels any pending auto-restart so the
		// process doesn't immediately come back.
		if c := st.sess.FindChild(action.childID); c != nil {
			c.SetAutoRestart(false)
		}
		_ = st.sess.Kill(action.childID, syscall.SIGTERM)
		st.repaintFocused()
		st.drawTabBar()
		st.drawSidebar()
		st.drawStatusLine()

	case "quit":
		st.requestExit()

	case "toasts-clear":
		if st.toasts.clear() {
			st.refreshToastSurface()
		}

	case "pad-delete":
		st.handlePadDelete(action.padName)

	case "pad-rename-submit":
		st.handlePadRename(action.padName, action.newName)

	case "pad-edit":
		st.handlePadEdit(action.padName)

	case "agent-rename-submit", "proc-rename-submit":
		st.handleChildRename(action.childID, action.newName)

	case "agent-close", "proc-delete":
		st.handleChildClose(action.childID, action.kind == "proc-delete")

	case "proc-stop":
		st.handleProcStop(action.childID)

	case "proc-restart":
		st.handleProcRestart(action.childID)

	case "settings-test":
		st.applySettingsAction(action)
		restoreView()
		st.drawTabBar()
		st.drawSidebar()
		st.drawStatusLine()
		go st.testSummarizer()

	case "settings-run-now":
		st.applySettingsAction(action)
		restoreView()
		st.drawTabBar()
		st.drawSidebar()
		st.drawStatusLine()
		st.runSummaryNow()
	}
}

func (st *uiState) applySettingsAction(action paletteAction) {
	if action.settings == nil {
		return
	}
	next := action.settings.clone()
	st.settingsMu.Lock()
	path := st.settingsPath
	st.settingsMu.Unlock()
	if err := saveSettings(path, next); err != nil {
		st.flashError(fmt.Sprintf("save settings: %v", err))
		return
	}
	st.settingsMu.Lock()
	st.settings = next
	st.settingsMu.Unlock()
}

func (st *uiState) testSummarizer() {
	if st.summaries == nil {
		return
	}
	base := st.ctx
	if base == nil {
		base = context.Background()
	}
	ctx, cancel := context.WithTimeout(base, summaryTimeout)
	defer cancel()
	if err := st.summaries.Test(ctx); err != nil {
		st.flashError(fmt.Sprintf("summarizer test: %v", err))
		return
	}
	st.flashTransient("summarizer test passed")
}

func (st *uiState) runSummaryNow() {
	if st.summaries == nil {
		return
	}
	st.mu.Lock()
	active := st.activeAgentID
	st.mu.Unlock()
	if active == "" {
		st.flashError("no active top-level agent to summarize")
		return
	}
	ctx := st.ctx
	if ctx == nil {
		ctx = context.Background()
	}
	st.summaries.RunNow(ctx, active)
	st.flashTransient("summary requested")
}

func (st *uiState) handlePadDelete(name string) {
	if name == "" || st.pads == nil {
		st.repaintFocused()
		return
	}
	if err := st.pads.Delete(name); err != nil {
		st.flashError(fmt.Sprintf("delete %s: %v", name, err))
		return
	}
	st.mu.Lock()
	if st.focusedPad == name {
		st.focusedPad = ""
	}
	st.mu.Unlock()
	st.scratchpadsChanged()
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

func (st *uiState) handlePadRename(oldName, newName string) {
	if oldName == "" || newName == "" || st.pads == nil {
		st.repaintFocused()
		return
	}
	if oldName == newName {
		st.repaintFocused()
		return
	}
	if err := st.pads.Rename(oldName, newName); err != nil {
		st.flashError(fmt.Sprintf("rename %s: %v", oldName, err))
		return
	}
	st.mu.Lock()
	if st.focusedPad == oldName {
		st.focusedPad = newName
	}
	st.mu.Unlock()
	st.scratchpadsChanged()
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// handlePadEdit launches an external editor (zed) on the focused
// scratchpad file. Fire-and-forget: we Start() the editor with
// stdin/stdout/stderr redirected to /dev/null and call Process.Release()
// so the patterm process doesn't accumulate zombies. The editor opens
// in its own window without suspending the TUI.
func (st *uiState) handlePadEdit(name string) {
	if name == "" || st.pads == nil {
		st.repaintFocused()
		return
	}
	path, err := st.pads.Path(name)
	if err != nil {
		st.flashError(fmt.Sprintf("edit %s: %v", name, err))
		return
	}
	null, err := os.OpenFile(os.DevNull, os.O_RDWR, 0)
	if err != nil {
		st.flashError(fmt.Sprintf("edit %s: open /dev/null: %v", name, err))
		return
	}
	cmd := exec.Command("zed", path)
	cmd.Stdin = null
	cmd.Stdout = null
	cmd.Stderr = null
	if err := cmd.Start(); err != nil {
		_ = null.Close()
		st.flashError(fmt.Sprintf("edit %s: %v", name, err))
		return
	}
	if cmd.Process != nil {
		_ = cmd.Process.Release()
	}
	_ = null.Close()
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

func (st *uiState) handleChildRename(childID, newName string) {
	if childID == "" || newName == "" {
		st.repaintFocused()
		return
	}
	c := st.sess.FindChild(childID)
	if c == nil {
		st.repaintFocused()
		return
	}
	c.SetName(newName)
	st.mu.Lock()
	if st.focusedID == childID {
		st.focusedName = newName
	}
	st.mu.Unlock()
	st.chromeCacheMu.Lock()
	st.tabBarCache = ""
	st.sidebarCache = ""
	st.chromeCacheMu.Unlock()
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// handleChildClose removes a child entry entirely. For agents this is
// equivalent to a SIGTERM kill (the entry is ephemeral and disappears
// from the session once the PTY exits). For command processes it's
// equivalent to the MCP close_process tool: SIGKILL if alive, then
// drop the entry so it stops appearing in the switch/restart lists.
func (st *uiState) handleChildClose(childID string, kill bool) {
	if childID == "" {
		st.repaintFocused()
		return
	}
	c := st.sess.FindChild(childID)
	if c == nil {
		st.repaintFocused()
		return
	}
	c.SetAutoRestart(false)
	if kill {
		_ = st.sess.Close(childID, syscall.SIGKILL)
	} else {
		_ = st.sess.Kill(childID, syscall.SIGTERM)
	}
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

func (st *uiState) handleProcStop(childID string) {
	if childID == "" {
		st.repaintFocused()
		return
	}
	c := st.sess.FindChild(childID)
	if c == nil {
		st.repaintFocused()
		return
	}
	c.SetAutoRestart(false)
	_ = st.sess.Kill(childID, syscall.SIGTERM)
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

func (st *uiState) handleProcRestart(childID string) {
	if childID == "" {
		st.repaintFocused()
		return
	}
	c := st.sess.FindChild(childID)
	if c == nil {
		st.repaintFocused()
		return
	}
	layout := st.layoutSnapshot()
	if err := st.sess.Restart(childID, syscall.SIGTERM, layout.childCols(), layout.childRows()); err != nil {
		st.flashError(fmt.Sprintf("restart %s: %v", c.DisplayName(), err))
		return
	}
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// flashError surfaces a spawn/etc. failure as an error toast over the
// focused pane. stderr is hidden under the alt screen so we can't rely
// on Fprintln(os.Stderr).
func (st *uiState) flashError(msg string) {
	st.notifyToast(toastError, msg)
}

// flashTransient is the softer cousin of flashError used for
// trust-prompt resolutions and other ack-style notices. Same
// stackable surface, info styling.
func (st *uiState) flashTransient(msg string) {
	st.notifyToast(toastInfo, msg)
}

// repaintFocused redraws the current focused child's screen snapshot.
// Callers must NOT hold st.mu — repaintFocused takes it
// briefly itself.
//
// We replay the emulator's padded grid snapshot rather than its VT
// serialization. SerializeVT can preserve style, but for diff-based TUIs
// we've seen it replay stale prompt layout that no longer matches the
// emulator grid; the padded snapshot is the source of truth for visible
// cells.
func (st *uiState) repaintFocused() {
	layout := st.layoutSnapshot()
	st.mu.Lock()
	id := st.focusedID
	renderer := st.renderer
	st.mu.Unlock()
	if id == "" {
		st.renderEmptyState()
		return
	}

	// Ratatui (codex) and other diff-based renderers can drift between
	// their internal "last frame" model and the emulator state when they
	// run unfocused, leaving incremental updates that target the wrong
	// cells after we replay. Nudge the focused child to redraw fully so
	// its next frame matches what we just put on the host.
	if c := st.sess.FindChild(id); c != nil && c.Status() == StatusRunning {
		cols, rows := layout.childCols(), layout.childRows()
		defer c.NudgeRedraw(cols, rows)
	}

	out := st.renderFocusedSnapshot(id, renderer, layout)
	if len(out) == 0 {
		return
	}
	st.mu.Lock()
	if st.focusedID == id {
		st.repaintNextPTY = id
		st.repaintNextPTYBudget = 2
	}
	st.mu.Unlock()
	st.outMu.Lock()
	_, _ = os.Stdout.Write(out)
	st.outMu.Unlock()
	st.renderToasts()
}

// repaintFocusedPad paints the focused scratchpad's content into the
// main viewport, honouring the per-pad scroll offset and clamping it
// to the rendered body size so a shrunk pad doesn't leave the view
// scrolled past its last line.
func (st *uiState) repaintFocusedPad() {
	st.mu.Lock()
	name := st.focusedPad
	st.mu.Unlock()
	if name == "" {
		return
	}
	layout := st.layoutSnapshot()
	content, _, err := st.pads.Read(name)
	if err != nil {
		content = fmt.Sprintf("(scratchpad %q unreadable: %v)", name, err)
	}
	out := st.renderPadView(name, content, layout)
	if len(out) == 0 {
		return
	}
	st.outMu.Lock()
	_, _ = os.Stdout.Write(out)
	st.outMu.Unlock()
	st.renderToasts()
}

// renderPadView builds the bytes that paint a scratchpad's content
// into the main viewport. Title row, divider, then a markdown-rendered
// body windowed by the per-pad scroll offset. Caller owns outMu and
// any prior clearViewportArea.
func (st *uiState) renderPadView(name, content string, layout terminalLayout) []byte {
	mainBottom := int(layout.statusRow) - statusRows
	width := int(layout.childCols())
	if mainBottom < int(layout.mainTop) || width < 1 {
		return nil
	}
	bodyCols := width - 1
	if bodyCols < 1 {
		bodyCols = 1
	}
	rendered := renderMarkdownLines(content, bodyCols)
	bodyRows := mainBottom - int(layout.mainTop) + 1 - 2
	if bodyRows < 1 {
		bodyRows = 1
	}
	maxOffset := len(rendered) - bodyRows
	if maxOffset < 0 {
		maxOffset = 0
	}
	st.mu.Lock()
	if st.padOffset > maxOffset {
		st.padOffset = maxOffset
	}
	if st.padOffset < 0 {
		st.padOffset = 0
	}
	offset := st.padOffset
	st.mu.Unlock()

	var b strings.Builder
	fmt.Fprintf(&b, "\x1b[0m\x1b[?6l\x1b[%d;%dr\x1b[?25l\x1b[%d;%dH",
		int(layout.mainTop), mainBottom,
		int(layout.mainTop), int(layout.mainLeft))

	row := int(layout.mainTop)
	writeRow := func(prefix, body, style string) {
		if row > mainBottom {
			return
		}
		fmt.Fprintf(&b, "\x1b[%d;%dH\x1b[%dX", row, int(layout.mainLeft), width)
		fmt.Fprintf(&b, "\x1b[%d;%dH%s", row, int(layout.mainLeft), style)
		b.WriteString(prefix)
		b.WriteString(body)
		b.WriteString(styleReset)
		row++
	}

	// Header tells the user which pad they're viewing and the scroll
	// position so a partial view is obvious.
	end := offset + bodyRows
	if end > len(rendered) {
		end = len(rendered)
	}
	title := fmt.Sprintf(" %s  (%d-%d / %d · ↑/↓ PgUp/PgDn · Esc back)",
		name, offset+1, end, len(rendered))
	if len(rendered) == 0 {
		title = fmt.Sprintf(" %s  (empty · Esc back)", name)
	}
	writeRow("", title, styleActive+styleBold)
	if width > 2 {
		writeRow("", " "+strings.Repeat("─", width-2), styleBorder)
	} else {
		writeRow("", strings.Repeat("─", width), styleBorder)
	}

	for i := offset; i < end; i++ {
		writeRow(" ", rendered[i], "")
	}
	for row <= mainBottom {
		writeRow("", "", "")
	}
	return []byte(b.String())
}

// exitPadView leaves scratchpad focus and falls back to the first
// running top-level child, or an empty viewport if there is none. No-op
// when no pad is focused.
func (st *uiState) exitPadView() {
	st.mu.Lock()
	if st.focusedPad == "" {
		st.mu.Unlock()
		return
	}
	st.focusedPad = ""
	st.focusedName = ""
	st.mu.Unlock()
	st.clearViewportArea()
	if next := firstRunningTopLevel(st.sess.Children()); next != nil {
		st.focusProcess(next.ID)
		return
	}
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// padScroll moves the focused-pad viewport by delta rows (negative =
// up, positive = down). No-op if no pad is focused. Clamping is
// performed against the rendered row count inside renderPadView, so
// callers can pass arbitrarily large step values for "jump to end".
func (st *uiState) padScroll(delta int) {
	st.mu.Lock()
	if st.focusedPad == "" {
		st.mu.Unlock()
		return
	}
	st.padOffset += delta
	if st.padOffset < 0 {
		st.padOffset = 0
	}
	st.mu.Unlock()
	st.repaintFocusedPad()
}

func (st *uiState) renderFocusedSnapshot(id string, renderer *viewportRenderer, layout terminalLayout) []byte {
	text, cursor, err := st.sess.SnapshotChild(id)
	if err != nil {
		return nil
	}
	if renderer != nil {
		if styled, err := st.sess.StyledSnapshotChild(id); err == nil && len(styled) > 0 {
			mainBottom := int(layout.statusRow) - statusRows
			prelude := fmt.Sprintf(
				"\x1b[0m\x1b[?6l\x1b[%d;%dr\x1b[?25h\x1b[%d;%dH",
				int(layout.mainTop), mainBottom,
				int(layout.mainTop), int(layout.mainLeft),
			)
			out := []byte(prelude)
			out = append(out, renderer.ClearViewport()...)
			out = append(out, renderer.Render(styled)...)
			cup := fmt.Sprintf("\x1b[%d;%dH", int(cursor.Row)+1, int(cursor.Col)+1)
			out = append(out, renderer.Render([]byte(cup))...)
			return out
		}
	}
	out := renderScreenSnapshot(text, cursor, layout)
	if renderer != nil {
		cup := fmt.Sprintf("\x1b[%d;%dH", int(cursor.Row)+1, int(cursor.Col)+1)
		out = append(out, renderer.Render([]byte(cup))...)
	}
	return out
}

func (st *uiState) requestExit() {
	// Reuse SIGTERM-to-self as the cleanest way to unwind: the signal
	// handler in Run() calls cancel() which exits the loop and runs
	// Shutdown.
	_ = syscall.Kill(os.Getpid(), syscall.SIGTERM)
}

func hostSize() (cols, rows uint16) {
	ws, err := cpty.GetsizeFull(os.Stdin)
	if err != nil || ws.Cols == 0 || ws.Rows == 0 {
		return 120, 40
	}
	return ws.Cols, ws.Rows
}