fix: zoom tears on a tall scroll region; terminal cursor follows the zoomed program

CHANGELOG.md

@@ -4,6 +4,21 @@ Every release is cut from an annotated tag whose message _is_ the release note,
 [GitHub releases](https://github.com/cmj0121/baton/releases) always carry the full story —
 the upgrade notes, the caveats, and why each change exists. This file is the index.
 
+## Unreleased
+
+- **A zoom no longer tears when a program sets a scroll region taller than the screen.** A program that still
+  believes an older, taller PTY size — every zoom passes through that window between its attach and its resize —
+  asked the emulator for margins past its last row, and the next delete-line panicked inside the emulator. The cockpit
+  recovered by dropping the rest of that chunk, so the panel showed a torn screen until the program repainted; the
+  daemon log carried the trace as `recovered an emulator write panic`. The margins are now clamped on the way in,
+  as a real terminal does (#139).
+- **The terminal's cursor follows the zoomed program.** The zoom painted a reverse-video cell where the program's
+  cursor was and left the real cursor at the end of the footer, which is where an input method draws the text it is
+  composing: CJK marked text there wrapped the line and scrolled the screen out from under the renderer. The zoom now
+  hands the terminal the program's cursor, so composition happens at the prompt, as in a plain terminal (#140).
+- **Wide glyphs in scrollback are clipped by cell.** A scrollback line with CJK text captured at a wider size was
+  clipped by rune count and could still overflow its tile by up to twice its width.
+
 ## [v2.4.2](https://github.com/cmj0121/baton/releases/tag/v2.4.2) — SYS follows the session you are in
 
 2026-09-23

internal/tui/clipcells_test.go

@@ -0,0 +1,33 @@
+package tui
+
+import (
+	"testing"
+
+	"github.com/charmbracelet/x/ansi"
+)
+
+// TestClipVisibleCounts cells, not runes: a scrollback line of CJK captured at a
+// wider size must come back no wider than its tile, or the terminal wraps it and
+// every row below shifts (#140).
+func TestClipVisibleCounts(t *testing.T) {
+	cases := []struct {
+		in    string
+		width int
+		want  string
+	}{
+		{"中文中文中文", 6, "中文中\x1b[0m"},
+		{"中文中文中文", 5, "中文\x1b[0m"}, // a wide glyph that would straddle the cut is dropped
+		{"ab中c", 4, "ab中\x1b[0m"},
+		{"\x1b[31m中文\x1b[0m中", 4, "\x1b[31m中文\x1b[0m\x1b[0m"},
+		{"abc", 5, "abc"}, // fits: no clip, no reset
+	}
+	for _, c := range cases {
+		got := clipVisible(c.in, c.width)
+		if got != c.want {
+			t.Errorf("clipVisible(%q, %d) = %q, want %q", c.in, c.width, got, c.want)
+		}
+		if w := ansi.StringWidth(got); w > c.width {
+			t.Errorf("clipVisible(%q, %d) is %d cells wide", c.in, c.width, w)
+		}
+	}
+}

internal/tui/margins_test.go

@@ -0,0 +1,42 @@
+package tui
+
+import (
+	"strings"
+	"testing"
+
+	vt "github.com/charmbracelet/x/vt"
+
+	"github.com/cmj0121/baton/internal/vtirm"
+)
+
+// TestWriteEmuSurvivesTallScrollRegion pins #139 at the choke point: a program that
+// still believes a taller PTY sets a scroll region past this emulator's last row,
+// then deletes a line inside it. Unclamped, the emulator indexed past its buffer,
+// the write panicked, and every byte after the delete in that chunk was lost —
+// here, the prompt. Clamped, the chunk lands whole.
+func TestWriteEmuSurvivesTallScrollRegion(t *testing.T) {
+	for _, op := range []string{"\x1b[M", "\x1b[L", "\x1bM", "\x1b[2S", "\x1b[2T"} {
+		emu := vt.NewSafeEmulator(80, 24)
+		irm := &vtirm.Filter{}
+		writeEmu(emu, irm, []byte("\x1b[1;40r\x1b[5;1Hold"))
+		writeEmu(emu, irm, []byte("\x1b[5;1H"+op+"\x1b[24;1H$ prompt"))
+		if !strings.Contains(emu.Render(), "$ prompt") {
+			t.Errorf("%q after a too-tall margin: the rest of the chunk was dropped", op)
+		}
+	}
+}
+
+// TestWriteEmuKeepsScrollRegionInside proves the clamp is a no-op for a region that
+// fits: the margins the program asked for are the ones the emulator scrolls within.
+func TestWriteEmuKeepsScrollRegionInside(t *testing.T) {
+	emu := vt.NewSafeEmulator(80, 24)
+	irm := &vtirm.Filter{}
+	writeEmu(emu, irm, []byte("\x1b[1;1Hkeep\x1b[2;24r\x1b[24;1Hbottom\n"))
+	rows := strings.Split(emu.Render(), "\n")
+	if !strings.HasPrefix(rows[0], "keep") {
+		t.Fatalf("row 0 = %q: the line above the region must not scroll", rows[0])
+	}
+	if !strings.HasPrefix(rows[22], "bottom") {
+		t.Fatalf("row 22 = %q: the region should have scrolled its bottom line up", rows[22])
+	}
+}

internal/tui/testdata/frames/zoom-scrolling.bin

@@ -1,7 +1,7 @@
 $ make test
 ok      baton/internal/tui      0.42s
 PASS
-$  
+$
 
 
 

internal/tui/testdata/frames/zoom.bin

@@ -1,7 +1,7 @@
 $ make test
 ok      baton/internal/tui      0.42s
 PASS
-$  
+$
 
 
 

internal/tui/tui.go

@@ -4076,6 +4076,9 @@ func (m model) cols() int {
 func (m model) View() tea.View {
 	v := tea.NewView(m.frame())
 	v.AltScreen = true
+	if x, y, ok := m.zoomCursor(); ok {
+		v.Cursor = tea.NewCursor(x, y)
+	}
 	if m.mouseEnabled {
 		// Cell motion reports clicks and the wheel without the noise of every
 		// pointer move, which is all the cockpit's wheel handling needs.
@@ -4229,20 +4232,38 @@ func (m model) zoomView() string {
 	lines := make([]string, rows)
 	if m.emu != nil {
 		lines = m.selectWindow(m.emu, m.width, rows, m.scrollOff)
-		// Draw the cursor only on the live bottom (a scrolled-back view is history),
-		// and only when the program has not hidden it — so a full-screen reader that
-		// turns the cursor off does not show a phantom block.
-		if m.scrollOff == 0 && !m.cursorHiddenNow() {
-			cur := m.emu.CursorPosition()
-			if cur.Y >= 0 && cur.Y < len(lines) {
-				lines[cur.Y] = overlayCursor(lines[cur.Y], cur.X)
-			}
-		}
 	}
 	footer := m.zoomFooter()
 	return strings.Join(lines, "\n") + "\n" + footer
 }
 
+// zoomCursor is where the terminal's own cursor belongs while a zoom is on screen:
+// on the cell the zoomed program's cursor is on, so the terminal composes there.
+// The zoom used to paint a reverse-video cell instead and leave the real cursor
+// wherever the renderer last wrote. That is where an input method draws the text
+// it is composing — the end of the footer — and CJK marked text there wraps the
+// line and scrolls the alternate screen out from under the diff renderer (#140).
+// A real terminal composes at the program's cursor; now so does the cockpit.
+//
+// No cursor — ok false, and the terminal keeps it hidden — outside a zoom, on a
+// cockpit too small to draw one, while scrolled back into history, while a copy
+// selection is being marked, while a text-input overlay (the search prompt) holds
+// the keyboard, or while the program has hidden it (DECTCEM), so a full-screen
+// reader that turns the cursor off does not show a phantom block.
+func (m model) zoomCursor() (x, y int, ok bool) {
+	if m.mode != modeZoom || m.emu == nil || m.width < minWidth || m.height < minHeight {
+		return 0, 0, false
+	}
+	if m.scrollOff != 0 || m.copySelecting || m.input != inputNone || m.cursorHiddenNow() {
+		return 0, 0, false
+	}
+	cur := m.emu.CursorPosition()
+	if cur.Y < 0 || cur.Y >= m.zoomRows() || cur.X < 0 || cur.X >= m.width {
+		return 0, 0, false
+	}
+	return cur.X, cur.Y, true
+}
+
 // dashboardView renders the status summary strip above the fleet. A small fleet
 // shows as a card grid; once it grows past treeThreshold it switches to a
 // space-efficient tree + preview split.

internal/tui/zoom.go

@@ -35,12 +35,21 @@ import (
 // bursts into the ICH the emulator implements, so a character typed into the middle of a
 // line pushes its neighbours right instead of landing on top of one. It carries state
 // across chunks, so each emulator needs its own; a nil filter leaves the stream alone.
+//
+// The filter is also told the emulator's height on every write, so a scroll region a
+// program sets for a taller screen than this one — the window between a zoom's attach and
+// its resize — is clamped before the emulator stores it. Left as asked, the next delete-line
+// indexes past the buffer; the recover below then drops the rest of the chunk and the panel
+// shows a torn screen until the program repaints (#139).
 func writeEmu(emu *vt.SafeEmulator, irm *vtirm.Filter, data []byte) {
 	defer func() {
 		if r := recover(); r != nil {
 			log.Error().Interface("panic", r).Bytes("stack", debug.Stack()).Msg("recovered an emulator write panic")
 		}
 	}()
+	if irm != nil {
+		irm.Rows = emu.Height()
+	}
 	_, _ = emu.Write(irm.Rewrite(vtquery.Strip(data)))
 }
 
@@ -195,6 +204,10 @@ func emuWindow(emu *vt.SafeEmulator, cols, rows, off int) []string {
 // copying escape sequences verbatim since they cost no columns. A scrollback line
 // captured at a wider size is thus trimmed to fit, and a trailing reset is added
 // when the clip lands mid-styling so a colour cannot bleed past the cut.
+//
+// Columns are display cells, not runes: a wide (CJK) glyph takes two, and one that
+// would straddle the cut is dropped rather than let the line run a cell over and
+// wrap in its tile (#140).
 func clipVisible(s string, width int) string {
 	if width < 1 {
 		return ""
@@ -208,13 +221,14 @@ func clipVisible(s string, width int) string {
 			i += n
 			continue
 		}
-		if vis >= width {
+		r, size := utf8.DecodeRuneInString(s[i:])
+		w := cellWidth(r)
+		if vis+w > width {
 			clipped = true
 			break
 		}
-		_, size := utf8.DecodeRuneInString(s[i:])
+		vis += w
 		out.WriteString(s[i : i+size])
-		vis++
 		i += size
 	}
 	if clipped {

internal/tui/zoom_test.go

@@ -214,18 +214,21 @@ func TestZoomTracksCursorVisibility(t *testing.T) {
 	}
 }
 
-// TestZoomViewHidesCursor proves the zoom view draws the cursor only while the
-// program keeps it visible.
+// TestZoomViewHidesCursor proves the zoom hands the terminal the program's cursor
+// only while the program keeps it visible — and never paints one of its own.
 func TestZoomViewHidesCursor(t *testing.T) {
-	emu := vt.NewSafeEmulator(20, 5)
+	emu := vt.NewSafeEmulator(80, 23)
 	hidden := false
-	m := model{emu: emu, mode: modeZoom, width: 20, height: 6, cursorHidden: &hidden}
-	if !strings.Contains(m.zoomView(), "\x1b[7m") {
-		t.Fatal("a visible cursor should be drawn")
+	m := model{emu: emu, mode: modeZoom, width: 80, height: 24, cursorHidden: &hidden}
+	if m.View().Cursor == nil {
+		t.Fatal("a visible cursor should be handed to the terminal")
 	}
-	hidden = true
 	if strings.Contains(m.zoomView(), "\x1b[7m") {
-		t.Fatal("a hidden cursor should not be drawn")
+		t.Fatal("the zoom should not paint a reverse-video cursor of its own")
+	}
+	hidden = true
+	if m.View().Cursor != nil {
+		t.Fatal("a hidden cursor should not be handed to the terminal")
 	}
 }
 

internal/tui/zoomcursor_test.go

@@ -0,0 +1,49 @@
+package tui
+
+import (
+	"testing"
+
+	vt "github.com/charmbracelet/x/vt"
+)
+
+// TestZoomCursorFollowsProgram proves the terminal cursor the zoom reports is the
+// zoomed program's own, cell for cell — including after a wide glyph, where a cursor
+// counted in runes would land a column short (#140).
+func TestZoomCursorFollowsProgram(t *testing.T) {
+	emu := vt.NewSafeEmulator(80, 23)
+	hidden := false
+	m := model{emu: emu, mode: modeZoom, width: 80, height: 24, cursorHidden: &hidden}
+	_, _ = emu.Write([]byte("\x1b[3;1H❯ hello 中文"))
+	cur := m.View().Cursor
+	if cur == nil {
+		t.Fatal("the zoom should report a cursor")
+	}
+	if cur.X != 12 || cur.Y != 2 {
+		t.Fatalf("cursor = (%d,%d), want (12,2): two wide glyphs take four cells", cur.X, cur.Y)
+	}
+}
+
+// TestZoomCursorWithheld lists the states in which the zoom must not report a
+// cursor: history (scrolled back), a copy selection, a text-input overlay that
+// holds the keyboard, the dashboard, a cockpit too small to draw a zoom at all.
+func TestZoomCursorWithheld(t *testing.T) {
+	emu := vt.NewSafeEmulator(80, 23)
+	hidden := false
+	base := model{emu: emu, mode: modeZoom, width: 80, height: 24, cursorHidden: &hidden}
+	cases := map[string]func(m model) model{
+		"scrolled back":   func(m model) model { m.scrollOff = 3; return m },
+		"copy selecting":  func(m model) model { m.copySelecting = true; return m },
+		"typing a search": func(m model) model { m.input = inputSearch; return m },
+		"not zoomed":      func(m model) model { m.mode = modeDashboard; return m },
+		"too small":       func(m model) model { m.width, m.height = 2, 2; return m },
+		"no emulator yet": func(m model) model { m.emu = nil; return m },
+	}
+	if base.View().Cursor == nil {
+		t.Fatal("the base zoom should report a cursor")
+	}
+	for name, tweak := range cases {
+		if tweak(base).View().Cursor != nil {
+			t.Errorf("%s: the zoom should withhold the cursor", name)
+		}
+	}
+}

internal/vtirm/margins_test.go

@@ -0,0 +1,52 @@
+package vtirm
+
+import (
+	"bytes"
+	"testing"
+)
+
+// TestClampMargins tables what a DECSTBM becomes on a 24-row emulator.
+func TestClampMargins(t *testing.T) {
+	cases := []struct {
+		in, want string
+	}{
+		{"\x1b[1;24r", "\x1b[1;24r"},   // fits: untouched
+		{"\x1b[r", "\x1b[r"},           // the whole screen: untouched
+		{"\x1b[1;40r", "\x1b[1;24r"},   // too tall: bottom pinned to the last row
+		{"\x1b[10;99r", "\x1b[10;24r"}, // top kept, bottom pinned
+		{"\x1b[30;40r", ""},            // wholly off screen: dropped, as xterm ignores it
+		{"\x1b[24;40r", ""},            // one row after the clamp: dropped
+		{"\x1b[0;40r", "\x1b[1;24r"},   // zero means default
+		{"\x1b[?1;40r", "\x1b[?1;40r"}, // a private sequence is not DECSTBM
+		{"\x1b[1;2;3r", "\x1b[1;2;3r"}, // too many parameters: not ours to judge
+		{"\x1b[1;40m", "\x1b[1;40m"},   // not a margin at all
+	}
+	for _, c := range cases {
+		f := &Filter{Rows: 24}
+		if got := f.Rewrite([]byte(c.in)); string(got) != c.want {
+			t.Errorf("Rewrite(%q) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}
+
+// TestClampMarginsNeedsRows proves a Filter that was never told a height leaves
+// margins alone rather than clamping to nothing.
+func TestClampMarginsNeedsRows(t *testing.T) {
+	f := &Filter{}
+	in := []byte("\x1b[1;40r")
+	if got := f.Rewrite(in); !bytes.Equal(got, in) {
+		t.Fatalf("Rewrite with Rows=0 = %q, want %q untouched", got, in)
+	}
+}
+
+// TestClampMarginsAcrossChunks proves a DECSTBM cut by a chunk boundary is still
+// clamped once its second half arrives, the same hold that keeps IRM honest.
+func TestClampMarginsAcrossChunks(t *testing.T) {
+	f := &Filter{Rows: 24}
+	var out []byte
+	out = append(out, f.Rewrite([]byte("\x1b[1;4"))...)
+	out = append(out, f.Rewrite([]byte("0r$ "))...)
+	if want := "\x1b[1;24r$ "; string(out) != want {
+		t.Fatalf("split DECSTBM = %q, want %q", out, want)
+	}
+}