Simplify session lifecycle and MCP cleanup

internal/app/app.go

@@ -105,6 +105,7 @@ func Run(ctx context.Context, opts Options) error {
 	host.attention = st
 	host.focus = st
 	host.prompter = st
+	host.scratch = st
 	st.lastExit.Store(-1)
 	sess.Subscribe(st)
 
@@ -241,10 +242,10 @@ type uiState struct {
 	// 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
+	chromeCacheMu   sync.Mutex
+	tabBarCache     string
+	sidebarCache    string
+	statusLineCache string
 
 	lastExit atomic.Int32
 }
@@ -278,10 +279,11 @@ func (st *uiState) focusProcess(processID string) {
 	if c == nil {
 		return
 	}
+	layout := st.layoutSnapshot()
 	st.mu.Lock()
 	st.focusedID = c.ID
 	st.focusedName = c.DisplayName()
-	st.renderer = newViewportRenderer(st.layoutSnapshot())
+	st.renderer = newViewportRenderer(layout)
 	st.mu.Unlock()
 	st.repaintFocused()
 	st.drawTabBar()
@@ -297,7 +299,7 @@ func (st *uiState) notifyAttention(childID, reason string) {
 	c := st.sess.FindChild(childID)
 	name := childID
 	if c != nil {
-		name = c.Name
+		name = c.DisplayName()
 	}
 	st.mu.Lock()
 	st.attentionText = fmt.Sprintf("attention: %s — %s", name, reason)
@@ -306,12 +308,20 @@ func (st *uiState) notifyAttention(childID, reason string) {
 	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.Name
-	renderer := newViewportRenderer(st.layoutSnapshot())
+	st.focusedName = c.DisplayName()
+	renderer := newViewportRenderer(layout)
 	st.renderer = renderer
 	palOpen := st.palette != nil
 	if palOpen {
@@ -343,17 +353,19 @@ func (st *uiState) OnChildSpawned(c *Child) {
 // 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 = ""
-			st.renderEmptyStateLocked()
+			renderEmpty = true
 		} else {
 			st.focusedID = next.ID
-			st.focusedName = next.Name
-			st.renderer = newViewportRenderer(st.layoutSnapshot())
+			st.focusedName = next.DisplayName()
+			st.renderer = newViewportRenderer(layout)
 		}
 	}
 	if st.palette != nil {
@@ -362,8 +374,12 @@ func (st *uiState) OnChildExited(c *Child) {
 		st.palette.rebuild()
 		st.renderPaletteLocked()
 	}
+	repaint := st.focusedID != ""
 	st.mu.Unlock()
-	if st.focusedID != "" {
+	if renderEmpty {
+		st.renderEmptyState()
+	}
+	if repaint {
 		st.repaintFocused()
 	}
 	st.drawTabBar()
@@ -417,13 +433,16 @@ func (st *uiState) OnPTYOut(childID string, chunk []byte) {
 	// contained one of those escapes; when set, drop the sidebar cache
 	// so the next drawSidebar repaints over the clobber instead of
 	// hitting the cache and leaving the gap visible.
-	if renderer.TookScrollAction() {
+	scrolled := renderer.TookScrollAction()
+	if scrolled {
 		st.chromeCacheMu.Lock()
 		st.sidebarCache = ""
 		st.chromeCacheMu.Unlock()
 	}
 	st.drawTabBar()
-	st.drawSidebar()
+	if scrolled {
+		st.drawSidebar()
+	}
 	st.drawStatusLine()
 }
 
@@ -559,15 +578,9 @@ func (st *uiState) drawStatusLine() {
 // renderEmptyState is the SPEC §4 blank-canvas hint. Drawn whenever no
 // child is focused.
 func (st *uiState) renderEmptyState() {
-	st.mu.Lock()
-	defer st.mu.Unlock()
-	st.renderEmptyStateLocked()
-}
-
-func (st *uiState) renderEmptyStateLocked() {
+	layout := st.layoutSnapshot()
 	st.outMu.Lock()
 	defer st.outMu.Unlock()
-	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)
@@ -897,10 +910,11 @@ func (st *uiState) closePalette(action paletteAction) {
 			st.repaintFocused()
 			return
 		}
+		layout := st.layoutSnapshot()
 		st.mu.Lock()
 		st.focusedID = action.childID
-		st.focusedName = c.Name
-		st.renderer = newViewportRenderer(st.layoutSnapshot())
+		st.focusedName = c.DisplayName()
+		st.renderer = newViewportRenderer(layout)
 		st.mu.Unlock()
 		st.repaintFocused()
 		st.drawTabBar()
@@ -953,10 +967,10 @@ func (st *uiState) flashTransient(msg string) {
 // 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
-	layout := st.layoutLocked()
 	st.mu.Unlock()
 	if id == "" {
 		st.renderEmptyState()