fix(tui): stop the preview stacking every repaint of a session

The overview showed the same session twice, one frame above another, after
switching sessions (reported from the beta with a screenshot).

Claude repaints by ABSOLUTE CURSOR POSITIONING, not by clearing: a 198KB pane
tail carries 1142 `CSI r;c H` and exactly one `CSI 2J`. The replay honoured the
COLUMN of those sequences and ignored the ROW, so a repaint could never
overwrite what came before and was appended instead. That same tail replayed as
FIFTY stacked copies of one frame. The preview shows the last N lines, so on a
short terminal you saw the newest frame by luck and on a tall one you saw the
end of the previous frame above it.

A cursor HOME now starts the buffer over. That is not a heuristic but the
line-based equivalent of what a home means: a full-screen app announcing it is
repainting from the top, with everything on screen about to be overwritten in
place. Only row 1 column 1 counts — any other address is a write position
inside the frame being painted, and resetting on those would erase live
content.

Measured on the real tail that produced the screenshot: 198599 bytes and 50
copies of the welcome frame collapse to 40 lines carrying exactly one.

The old test pinned the append behaviour, including a spurious leading empty
line that the initial CUP produced; both are gone.

src/tui/tui-ansi.ts

@@ -51,13 +51,18 @@ interface EscapeScan {
   sgr?: string;
   /** 1-based column of a cursor-position sequence (`CSI r ; c H` or `f`). */
   column?: number;
+  /** 1-based row of that same sequence. Row 1 means a repaint is starting. */
+  row?: number;
 }
 
-/** The column a `CSI r ; c H` addresses. Both parameters default to 1. */
-function cursorColumn(params: string): number {
+/** The row and column a `CSI r ; c H` addresses. Both parameters default to 1. */
+function cursorPosition(params: string): { row: number; column: number } {
   const parts = params.split(';');
-  const column = Number.parseInt(parts[1] ?? '', 10);
-  return Number.isSafeInteger(column) && column > 0 ? column : 1;
+  const read = (index: number): number => {
+    const value = Number.parseInt(parts[index] ?? '', 10);
+    return Number.isSafeInteger(value) && value > 0 ? value : 1;
+  };
+  return { row: read(0), column: read(1) };
 }
 
 /** Scan a CSI body starting at `from` (params, then intermediates, then a final byte). */
@@ -69,7 +74,7 @@ function readCsi(text: string, start: number, from: number, keepSgr: boolean): E
   const next = j + 1;
   if (keepSgr && text[j] === 'm') return { next, sgr: text.slice(start, next) };
   if (keepSgr && (text[j] === 'H' || text[j] === 'f')) {
-    return { next, column: cursorColumn(text.slice(from, j)) };
+    return { next, ...cursorPosition(text.slice(from, j)) };
   }
   return { next };
 }
@@ -423,6 +428,18 @@ export function toDisplayLines(raw: string): string[] {
       if (scan.sgr !== undefined) {
         active = applySgr(active, scan.sgr);
         sgr = active.join('');
+      } else if (scan.row === 1 && scan.column === 1) {
+        // ⚠️ A HOME is a full-screen app announcing that it is repainting from
+        // the top, and everything already on screen is about to be overwritten
+        // in place. This replay is line-based and cannot overwrite, so the
+        // faithful equivalent is to start over — without it every repaint was
+        // APPENDED, and a claude pane's tail carried fifty stacked copies of
+        // the same frame. The preview then showed the last N lines, which on a
+        // tall terminal spanned two of them (reported from the beta as the
+        // overview showing the session twice).
+        lines.length = 0;
+        cells = [];
+        col = 0;
       } else if (scan.column !== undefined) {
         // Column 1 is a fresh row, which is the only thing a repainting TUI
         // gives us to split lines on.

test/tui/tui-ansi.test.ts

@@ -54,15 +54,30 @@ describe('toDisplayLines', () => {
   it('splits a row-addressed repaint into lines, which is how an Ink TUI paints', () => {
     // Claude Code emits almost no newlines: without this the whole screen is
     // one line and nothing in the preview is readable.
-    expect(toDisplayLines('\x1b[1;1Hfirst\x1b[2;1Hsecond\x1b[3;1Hthird')).toEqual(['', 'first', 'second', 'third']);
+    expect(toDisplayLines('\x1b[1;1Hfirst\x1b[2;1Hsecond\x1b[3;1Hthird')).toEqual(['first', 'second', 'third']);
     // A jump inside a row is a write position, not a new line.
-    expect(toDisplayLines('\x1b[1;1Hab\x1b[1;5Hcd')).toEqual(['', 'ab  cd']);
-    expect(toDisplayLines('\x1b[1;1Habcdef\x1b[1;2HXY')).toEqual(['', 'aXYdef']);
-    // Both parameters default to 1, so a bare CUP is a fresh row.
-    expect(toDisplayLines('a\x1b[Hb')).toEqual(['a', 'b']);
+    expect(toDisplayLines('\x1b[1;1Hab\x1b[1;5Hcd')).toEqual(['ab  cd']);
+    expect(toDisplayLines('\x1b[1;1Habcdef\x1b[1;2HXY')).toEqual(['aXYdef']);
     expect(toDisplayLines('a\x1b[3;1fb')).toEqual(['a', 'b']);
   });
 
+  it('starts a new frame at a cursor HOME, instead of stacking repaints', () => {
+    // ⚠️ A home is a full-screen app announcing a repaint from the top, and
+    // everything on screen is about to be overwritten in place. This replay is
+    // line-based and cannot overwrite, so starting over is the faithful
+    // equivalent. Without it every repaint was APPENDED: a real claude pane's
+    // 198KB tail replayed as FIFTY stacked copies of the same frame, and the
+    // preview showed the last N lines, which on a tall terminal spanned two of
+    // them — the overview appeared to show the session twice.
+    expect(toDisplayLines('old frame\x1b[1;1Hnew frame')).toEqual(['new frame']);
+    // Both parameters default to 1, so a bare CUP is a home too.
+    expect(toDisplayLines('a\x1b[Hb')).toEqual(['b']);
+    // Only row 1 column 1. Any other address is a write position within the
+    // frame being painted, and resetting on those would erase live content.
+    expect(toDisplayLines('keep\x1b[2;1Hnext')).toEqual(['keep', 'next']);
+    expect(toDisplayLines('keep\x1b[1;3Hxx')).toEqual(['kexx']);
+  });
+
   it('refuses to allocate a line for a column no terminal has', () => {
     const lines = toDisplayLines('\x1b[1;99999Hx');
     expect(lines).toHaveLength(1);