test(terminal): exercise native synchronized-output erase handling

Read xterm's synchronized-output mode in the erase handler itself, remove
unused custom tracking, and cover combined modes and split writes.

Co-authored-by: si1enteee <si1enteee@users.noreply.github.com>

components/terminal/clearTerminalViewport.test.ts

@@ -66,6 +66,7 @@ const createEraseHandlerHarness = (
   };
   const observedScrollRegions: Array<[number, number]> = [];
   const term = {
+    modes: { synchronizedOutputMode: options.inDec2026SyncBlock ?? false },
     rows: 5,
     options: {
       scrollOnEraseInDisplay: false,
@@ -118,7 +119,6 @@ const createEraseHandlerHarness = (
   };
   const disposable = installEraseInDisplayHandlers(term as never, {
     getClearWipesScrollback: () => options.clearWipesScrollback ?? false,
-    isInDec2026SyncBlock: () => options.inDec2026SyncBlock ?? false,
     scheduleMicrotask: (callback) => {
       microtasks.push(callback);
     },
@@ -321,7 +321,6 @@ test("erase-below wipe preserves later output from the same write batch", async
   const term = new Terminal({ cols: 20, rows: 5, scrollback: 100 });
   const disposable = installEraseInDisplayHandlers(term as never, {
     getClearWipesScrollback: () => true,
-    isInDec2026SyncBlock: () => false,
   });
 
   await writeTerminal(term, "old1\r\nold2\r\nold3\r\nold4\r\nold5\r\nold6\r\nold7\r\nold8");
@@ -524,3 +523,96 @@ test("appendEraseScrollback still wipes when delayed clear is outside a sync blo
     "\x1b[H\x1b[2J\x1b[3Jframe",
   );
 });
+
+const getScrollbackLines = (term: InstanceType<typeof Terminal>): string[] =>
+  Array.from({ length: term.buffer.active.baseY }, (_, row) =>
+    term.buffer.active.getLine(row)?.translateToString(true) ?? ""
+  );
+
+test("native synchronized-output mode ignores unrelated private modes", async () => {
+  const term = new Terminal({ cols: 20, rows: 5, scrollback: 100 });
+
+  await writeTerminal(term, "\x1b[?2004h");
+  assert.equal(term.modes.synchronizedOutputMode, false, "bracketed paste 2004 must not open a sync block");
+
+  await writeTerminal(term, "\x1b[?25h\x1b[?1049h");
+  assert.equal(term.modes.synchronizedOutputMode, false, "cursor/alt-screen modes must not open a sync block");
+
+  await writeTerminal(term, "\x1b[?25:2026h");
+  assert.equal(term.modes.synchronizedOutputMode, false, "subparameters do not enable sync mode");
+
+  await writeTerminal(term, "\x1b[?25;2026h");
+  assert.equal(term.modes.synchronizedOutputMode, true, "combined primary modes enable sync mode");
+
+  await writeTerminal(term, "\x1b[?2004l");
+  assert.equal(term.modes.synchronizedOutputMode, true, "closing 2004 must not close the 2026 block");
+
+  await writeTerminal(term, "\x1b[?2026l");
+  assert.equal(term.modes.synchronizedOutputMode, false);
+
+  term.dispose();
+});
+
+test("Ctrl+L clear survives a bash prompt (2004 open, 2026 closed)", async () => {
+  const term = new Terminal({ cols: 20, rows: 5, scrollback: 100 });
+  const erase = installEraseInDisplayHandlers(term as never, {
+    getClearWipesScrollback: () => false,
+  });
+
+  // Exactly two rows already in scrollback, then a full screen of visible rows.
+  await writeTerminal(term, "old1\r\nold2\r\nvis1\r\nvis2\r\nvis3\r\nvis4\r\nvis5");
+  assert.equal(term.buffer.active.baseY, 2);
+
+  // bash 5.1 at a prompt holds bracketed paste open; Ctrl+L only sends CSI 2 J.
+  await writeTerminal(term, "\x1b[?2004h");
+  await writeTerminal(term, "\x1b[H\x1b[2Jnew1");
+
+  const scrollback = getScrollbackLines(term);
+  assert.equal(scrollback.some((line) => line.startsWith("vis")), true, "visible rows preserved in scrollback");
+  assert.equal(scrollback.some((line) => line.startsWith("old")), true, "earlier history not trimmed");
+
+  erase.dispose();
+  term.dispose();
+});
+
+test("clears inside a DEC 2026 sync block still erase in place", async () => {
+  const term = new Terminal({ cols: 20, rows: 5, scrollback: 100 });
+  const erase = installEraseInDisplayHandlers(term as never, {
+    getClearWipesScrollback: () => false,
+  });
+
+  await writeTerminal(term, "old1\r\nold2\r\nvis1\r\nvis2\r\nvis3\r\nvis4\r\nvis5");
+  assert.equal(term.buffer.active.baseY, 2);
+  await writeTerminal(term, "\x1b[?2026h\x1b[H\x1b[2Jnew1\x1b[?2026l");
+
+  const scrollback = getScrollbackLines(term);
+  assert.equal(scrollback.some((line) => line.startsWith("vis")), false, "sync-block erase stays in place");
+  assert.equal(scrollback.some((line) => line.startsWith("old")), true, "only pre-existing scrollback remains");
+
+  erase.dispose();
+  term.dispose();
+});
+
+for (const [label, sequence, preservesViewport] of [
+  ["combined primary modes", "\x1b[?25;2026h", false],
+  ["subparameter only", "\x1b[?25:2026h", true],
+  ["explicit reset", "\x1b[?2026h\x1b[?2026l", true],
+] as const) {
+  test(`full erase uses native synchronized-output state for ${label}`, async () => {
+    const term = new Terminal({ cols: 20, rows: 5, scrollback: 100 });
+    const erase = installEraseInDisplayHandlers(term, {
+      getClearWipesScrollback: () => false,
+    });
+    try {
+      await writeTerminal(term, "old1\r\nold2\r\nvis1\r\nvis2\r\nvis3\r\nvis4\r\nvis5");
+      // The mode and erase arrive in separate PTY chunks, as with TUI redraws.
+      await writeTerminal(term, sequence);
+      await writeTerminal(term, "\x1b[H\x1b[2Jnew1");
+      assert.equal(getScrollbackLines(term).some((line) => line.startsWith("vis")), preservesViewport);
+      assert.equal(getScrollbackLines(term).some((line) => line.startsWith("old")), true);
+    } finally {
+      erase.dispose();
+      term.dispose();
+    }
+  });
+}

components/terminal/clearTerminalViewport.ts

@@ -47,7 +47,6 @@ type AppendEraseScrollbackOptions = {
 
 type EraseInDisplayHandlerOptions = {
   getClearWipesScrollback: () => boolean;
-  isInDec2026SyncBlock: () => boolean;
   scheduleMicrotask?: (callback: () => void) => void;
 };
 
@@ -264,7 +263,6 @@ export const installEraseInDisplayHandlers = (
   term: EraseInDisplayTerminal,
   {
     getClearWipesScrollback,
-    isInDec2026SyncBlock,
     scheduleMicrotask = queueMicrotask,
   }: EraseInDisplayHandlerOptions,
 ): IDisposable => {
@@ -279,7 +277,8 @@ export const installEraseInDisplayHandlers = (
 
   const eraseDisposable = term.parser.registerCsiHandler({ final: "J" }, (params) => {
     const wipeAllowed = getClearWipesScrollback();
-    const inDec2026SyncBlock = isInDec2026SyncBlock();
+    // Use xterm's parsed mode, including its native safety timeout.
+    const inDec2026SyncBlock = term.modes.synchronizedOutputMode;
     // Scope xterm's native preservation to shell clears, not TUI redraws.
     if (isEraseViewportSequence(params)) {
       const useNativeScrollPreservation = shouldScrollOnEraseInDisplay(

components/terminal/runtime/createXTermRuntime.ts

@@ -3170,28 +3170,8 @@ export const createXTermRuntime = (ctx: CreateXTermRuntimeContext): XTermRuntime
   // OSC 7 format: \x1b]7;file://hostname/path\x07 or \x1b]7;file://hostname/path\x1b\\
   let currentCwd: string | undefined = undefined;
 
-  // Track DEC 2026 synchronized-output blocks so CSI 2 J can erase in place for
-  // Codex/Claude Code TUIs instead of pushing visible rows into scrollback.
-  let inDec2026SyncBlock = false;
-
-  const dec2026SyncStartDisposable = term.parser.registerCsiHandler(
-    { prefix: "?", final: "h", params: [2026] },
-    () => {
-      inDec2026SyncBlock = true;
-      return false;
-    },
-  );
-  const dec2026SyncEndDisposable = term.parser.registerCsiHandler(
-    { prefix: "?", final: "l", params: [2026] },
-    () => {
-      inDec2026SyncBlock = false;
-      return false;
-    },
-  );
-
   const eraseScrollbackDisposable = installEraseInDisplayHandlers(term, {
     getClearWipesScrollback: () => ctx.terminalSettingsRef.current?.clearWipesScrollback ?? true,
-    isInDec2026SyncBlock: () => inDec2026SyncBlock,
   });
 
   const markCursorPositionReportRequest = (params: readonly (number | number[])[]): boolean => {
@@ -3500,8 +3480,6 @@ export const createXTermRuntime = (ctx: CreateXTermRuntimeContext): XTermRuntime
       pluginLinkProviderHost?.dispose();
       pluginProviderHost?.dispose();
       eraseScrollbackDisposable.dispose();
-      dec2026SyncStartDisposable.dispose();
-      dec2026SyncEndDisposable.dispose();
       for (const disposable of cursorPositionReportRequestDisposables) {
         disposable.dispose();
       }