patterm/internal/app/app.go

package app

import (
	"context"
	"errors"
	"fmt"
	"io"
	"os"
	"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/preset"
	"github.com/hjbdev/patterm/internal/scratchpad"
	"github.com/hjbdev/patterm/internal/trust"
)

// Options configures a patterm run.
type Options struct {
	ProjectDir string
	ProjectKey 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)
	}

	// 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)
	}

	// 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()

	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()

	st := &uiState{
		sess:     sess,
		presets:  presets,
		launcher: launcher,
		pads:     pads,
		trust:    trustStore,
		hostCols: cols,
		hostRows: rows,
		stdinTTY: term.IsTerminal(int(os.Stdin.Fd())),
	}
	host.attention = st
	host.focus = st
	host.prompter = st
	host.scratch = st
	st.lastExit.Store(-1)
	sess.Subscribe(st)

	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())

	var wg sync.WaitGroup

	// SIGWINCH.
	wg.Add(1)
	winch := make(chan os.Signal, 1)
	signal.Notify(winch, syscall.SIGWINCH)
	go func() {
		defer wg.Done()
		defer signal.Stop(winch)
		for {
			select {
			case <-ctx.Done():
				return
			case <-winch:
				c, r := hostSize()
				if c == 0 || r == 0 {
					continue
				}
				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.repaintFocused()
				st.drawTabBar()
				st.drawSidebar()
				st.drawStatusLine()
			}
		}
	}()

	// 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

	outMu sync.Mutex

	mu          sync.Mutex
	palette     *paletteState
	focusedID   string
	focusedName 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

	// attention is the latest request_human_attention surfaced via MCP;
	// rendered in the status line until cleared.
	attentionText string
	attentionAt   string

	// 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

	// 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

	lastExit atomic.Int32
}

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

// 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
	}
	layout := st.layoutSnapshot()
	st.mu.Lock()
	st.focusedID = c.ID
	st.focusedName = c.DisplayName()
	st.updateActiveAgentLocked(c)
	st.renderer = newViewportRenderer(layout)
	st.mu.Unlock()
	st.repaintFocused()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

func (st *uiState) restartFocusedCommand(processID string) {
	c := st.sess.FindChild(processID)
	if c == nil || c.Kind != KindCommand {
		return
	}
	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 = 8
	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
// surface a one-line toast in the status row and remember the most
// recent ask so the status line keeps showing it. 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.mu.Lock()
	st.attentionText = fmt.Sprintf("attention: %s — %s", name, reason)
	st.attentionAt = childID
	st.mu.Unlock()
	st.drawStatusLine()
}

func (st *uiState) scratchpadsChanged() {
	st.chromeCacheMu.Lock()
	st.sidebarCache = ""
	st.chromeCacheMu.Unlock()
	st.drawSidebar()
}

// OnChildSpawned auto-focuses the new child.
func (st *uiState) OnChildSpawned(c *Child) {
	layout := st.layoutSnapshot()
	st.mu.Lock()
	st.focusedID = c.ID
	st.focusedName = c.DisplayName()
	st.updateActiveAgentLocked(c)
	renderer := newViewportRenderer(layout)
	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 = 8
	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.moveToViewportOrigin()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// OnChildExited drops focus and shows the empty state if it was the
// focused child.
func (st *uiState) OnChildExited(c *Child) {
	st.lastExit.Store(int32(c.ExitCode()))
	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) {
	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 {
		return
	}
	var out []byte
	if forceRepaint {
		out = st.renderFocusedSnapshot(childID, renderer, layout)
		if len(out) == 0 {
			return
		}
	} else {
		out = renderer.Render(chunk)
	}
	st.outMu.Lock()
	_, _ = os.Stdout.Write([]byte("\x1b[?7l"))
	_, _ = os.Stdout.Write(out)
	_, _ = os.Stdout.Write([]byte("\x1b[?7h"))
	st.outMu.Unlock()
	// 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()
	}
	st.drawTabBar()
	if scrolled {
		st.drawSidebar()
	}
	st.drawStatusLine()
}

func (st *uiState) enterScreen() {
	st.outMu.Lock()
	defer st.outMu.Unlock()
	_, _ = os.Stdout.Write([]byte("\x1b[?1049h\x1b[H\x1b[2J\x1b[?25h"))
}

func (st *uiState) leaveScreen() {
	st.outMu.Lock()
	defer st.outMu.Unlock()
	_, _ = os.Stdout.Write([]byte("\x1b[?25h\x1b[?1049l"))
}

func (st *uiState) clearScreen() {
	st.invalidateChromeCache()
	st.outMu.Lock()
	defer st.outMu.Unlock()
	_, _ = os.Stdout.Write([]byte("\x1b[?25h\x1b[H\x1b[2J"))
}

// 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.
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() {
	st.mu.Lock()
	palOpen := st.palette != nil
	focusID := st.focusedID
	focusName := st.focusedName
	attention := st.attentionText
	attentionAt := st.attentionAt
	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
	}
	owner := ""
	if focusID != "" {
		if c := st.sess.FindChild(focusID); c != nil {
			switch c.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 attention != "" && attentionAt == focusID {
		left = "[!] " + attention
	}
	if attention != "" && attentionAt == "" {
		// Sticky attention/flash from somewhere outside the focused pane.
		left = "[!] " + attention
	}
	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 c := st.sess.FindChild(focusID); c != nil && c.Kind == KindCommand {
		hints = append(hints, "Ctrl-R · restart")
	}
	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()
		return
	}
	st.statusLineCache = line
	st.chromeCacheMu.Unlock()

	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()
	st.outMu.Lock()
	defer st.outMu.Unlock()
	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)
	}
	fmt.Fprintf(os.Stdout, "\x1b[?25l\x1b[H\x1b[2J\x1b[%d;%dH\x1b[2m%s\x1b[0m", row, col, line)
}

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))
	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 prev != OwnerUser {
					go st.drawStatusLine()
				}
			}
		}
		forward = forward[:0]
	}

	var pendingAction *paletteAction
	var pendingNavID string
	var pendingRestartID string

	// 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]

		// 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 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()
			pendingNavID = nextTabID(st.sess.Children(), st.focusedID, -1)
			i += adv
			break
		}
		if hit, adv := matchCtrlChar(chunk, i, 'd'); hit {
			flushForward()
			pendingNavID = nextTabID(st.sess.Children(), st.focusedID, +1)
			i += adv
			break
		}
		if hit, adv := matchCtrlChar(chunk, i, 'w'); hit {
			flushForward()
			pendingNavID = nextChildID(st.sess.Children(), st.focusedID, st.activeAgentID, -1)
			i += adv
			break
		}
		if hit, adv := matchCtrlChar(chunk, i, 's'); hit {
			flushForward()
			pendingNavID = nextChildID(st.sess.Children(), st.focusedID, st.activeAgentID, +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
			}
		}

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

	if pendingAction != nil {
		st.closePalette(*pendingAction)
	}
	if pendingNavID != "" {
		st.focusProcess(pendingNavID)
	}
	if pendingRestartID != "" {
		st.restartFocusedCommand(pendingRestartID)
	}
}

func (st *uiState) openPaletteLocked() {
	st.palette = newPalette(st.sess.Children(), st.focusedID, st.presets)
	// 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()

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

	case "spawn-agent":
		if action.preset == nil {
			st.repaintFocused()
			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 {
			st.repaintFocused()
			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-process-submit":
		if action.command == "" {
			st.repaintFocused()
			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) {
			st.repaintFocused()
			return
		}
		layout := st.layoutSnapshot()
		st.mu.Lock()
		st.focusedID = action.childID
		st.focusedName = c.DisplayName()
		st.updateActiveAgentLocked(c)
		st.renderer = newViewportRenderer(layout)
		st.mu.Unlock()
		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()
	}
}

// flashError surfaces a spawn/etc. failure in the status line until the
// next attention update overwrites it. stderr is hidden under the alt
// screen so we can't rely on Fprintln(os.Stderr).
func (st *uiState) flashError(msg string) {
	st.mu.Lock()
	st.attentionText = msg
	st.attentionAt = "" // shows on every focus until cleared
	st.mu.Unlock()
	st.renderEmptyState()
	st.drawTabBar()
	st.drawSidebar()
	st.drawStatusLine()
}

// flashTransient is the softer cousin of flashError used for
// trust-prompt resolutions. Same status-line surface; the prefix differs.
func (st *uiState) flashTransient(msg string) {
	st.mu.Lock()
	st.attentionText = msg
	st.attentionAt = ""
	st.mu.Unlock()
	st.drawStatusLine()
}

// 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 = 8
	}
	st.mu.Unlock()
	st.outMu.Lock()
	defer st.outMu.Unlock()
	_, _ = os.Stdout.Write(out)
}

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
}