Fix scroll off-by-one in synthetic flood; guard repro against buffer scroll

generateColoredAsciiFlood emitted a trailing \r\n after its final cell, so a
flood whose last line lands on the bottom row scrolled the screen. In the
alt-buffer repro this destroyed the pinned header row in the BUFFER, making
the 255/255 signature diff a content change, not renderer corruption. The
single-terminal merge path is actually healthy: with the scroll removed the
same 30-chunk merge-heavy workload renders the header with zero drift.

- generateColoredAsciiFlood: no linefeed after the final cell
- WebglReproExperiment: now a green guard; asserts row-1 buffer text first so
  a content bug can never masquerade as corruption again
- WebglVisual: accurate comments; erase rewritten lines so leftover prefill
  text doesn't look like corruption
- WebglSingleTermStress: replace stale EXPECTED-RED comment (test passes)

addons/addon-webgl/test/WebglReproExperiment.test.ts

@@ -1,32 +1,43 @@
 /**
  * Copyright (c) 2025 The xterm.js authors. All rights reserved.
  * @license MIT
- * EXPERIMENT: try to reproduce vscode#322756 garbled single-terminal render.
+ *
+ * Single-terminal guard for the vscode#322756 "garbled glyph" class: a pinned
+ * alt-buffer header must render identically while the body churns enough
+ * distinct colored glyphs to force texture-atlas page merges.
+ *
+ * Note this scenario is GREEN on current master: the single-terminal mid-frame
+ * merge race was fixed upstream (merge-retry loop in WebglRenderer.renderRows +
+ * globally monotonic AtlasPage versions). The test asserts the buffer content
+ * first so a content bug (e.g. an accidental scroll from the churn stream) can
+ * never masquerade as renderer corruption.
  */
 import test, { expect } from '@playwright/test';
 import { ITestContext, createTestContext, openTerminal } from '../../../test/playwright/TestUtils';
-import { captureRowSignatures, expectSignatureMatches, getAtlasPageCount, loadWebglStrict, setMaxAtlasPages, waitForRender, writeAndWaitForRender } from '../../../test/playwright/RendererTestUtils';
+import { captureRowSignatures, expectSignatureMatches, getAtlasPageCount, loadWebglStrict, setMaxAtlasPages, writeAndWaitForRender } from '../../../test/playwright/RendererTestUtils';
 import { generateColoredAsciiFlood } from '../../../test/playwright/SyntheticTui';
 
+const HEADER = ' HEADER_REF_0123456789';
+
 let ctx: ITestContext;
 test.beforeAll(async ({ browser }) => { ctx = await createTestContext(browser); });
 test.afterAll(async () => await ctx.page.close());
 
-test.describe('repro #322756', () => {
+test.describe('single-terminal merge integrity (#322756 class)', () => {
   test.skip(({ browserName }) => browserName !== 'chromium');
   test.describe.configure({ timeout: 60000 });
 
-  test('header survives merge race with per-chunk sync', async () => {
+  test('pinned header renders identically across 30 merge-heavy body redraws', async () => {
     await openTerminal(ctx, { cols: 80, rows: 24 });
     await loadWebglStrict(ctx);
     await setMaxAtlasPages(ctx, 4);
     const cols = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
     // Alt-buffer TUI: pinned header on row 1, body churns below (like Copilot CLI).
-    await writeAndWaitForRender(ctx, '\x1b[?1049h\x1b[H HEADER_REF_0123456789');
+    await writeAndWaitForRender(ctx, `\x1b[?1049h\x1b[H${HEADER}`);
     const ref = await captureRowSignatures(ctx, 1, cols);
     let maxDiff = 0;
     for (let c = 0; c < 30; c++) {
-      // redraw header + churn body rows 2..24 with unique glyphs, header stays put
+      // Redraw body rows 2..24 with unique glyphs; the header row is never touched.
       let frame = '\x1b[2;1H';
       frame += generateColoredAsciiFlood(23 * 78, c * 23 * 78);
       await writeAndWaitForRender(ctx, frame);
@@ -37,6 +48,12 @@ test.describe('repro #322756', () => {
       }
     }
     console.error('[repro] pages=', await getAtlasPageCount(ctx), 'headerMaxDiff=', maxDiff);
+
+    // Guard: the header must still be in the buffer. If this fails the churn
+    // stream scrolled the screen and any pixel diff is meaningless.
+    const row1Text = await ctx.page.evaluate(() => (window as any).term.buffer.active.getLine(0)?.translateToString(true));
+    expect(row1Text, 'churn stream must not scroll the pinned header out of the buffer').toBe(HEADER);
+
     const after = await captureRowSignatures(ctx, 1, cols);
     for (let i = 0; i < cols.length; i++) {
       expectSignatureMatches(after[i], ref[i], 14, `header col ${cols[i]} garbled`);