fix: clear scrolled row cells in wasm patch

lib/viewport-corruption.test.ts

@@ -0,0 +1,194 @@
+/**
+ * Regression coverage for stale cells becoming visible after scroll growth.
+ *
+ * The bug requires ESC[0m reset-heavy output, default cursor background,
+ * repeated scrolls, and rows that are not fully overwritten.
+ */
+
+import { describe, expect, test } from 'bun:test';
+import type { Terminal } from './terminal';
+import { createIsolatedTerminal } from './test-helpers';
+import type { GhosttyCell } from './types';
+
+const ESC = '\x1b';
+const MARKER_PATTERN = /R\d{2}L\d{3}/g;
+
+function buildStressPayload(): Uint8Array {
+  const lines: string[] = [];
+  let lineNumber = 0;
+
+  const pushMarkedLine = (content: string): void => {
+    const marker = `R00L${lineNumber.toString().padStart(3, '0')}`;
+    lines.push(`${ESC}[38;5;${(lineNumber * 17) % 256}m${marker}${ESC}[0m ${content}`);
+    lineNumber++;
+  };
+
+  const pushBlankLine = (): void => {
+    lines.push('');
+    lineNumber++;
+  };
+
+  pushMarkedLine(`${ESC}[1m${'═'.repeat(72)}${ESC}[0m`);
+  pushMarkedLine(`${ESC}[1mTerminal rendering scroll-growth regression${ESC}[0m`);
+  pushBlankLine();
+
+  pushMarkedLine(`${ESC}[1m256-color palette${ESC}[0m`);
+  for (let row = 0; row < 8; row++) {
+    let content = '';
+    for (let i = 0; i < 32; i++) {
+      content += `${ESC}[48;5;${row * 32 + i}m  ${ESC}[0m`;
+    }
+    pushMarkedLine(content);
+  }
+  pushBlankLine();
+
+  pushMarkedLine(`${ESC}[1mTruecolor gradients${ESC}[0m`);
+  for (let row = 0; row < 8; row++) {
+    let content = '';
+    for (let i = 0; i < 80; i++) {
+      const r = Math.floor(Math.sin(i * 0.08 + row) * 127 + 128);
+      const g = Math.floor(Math.sin(i * 0.08 + row + 2) * 127 + 128);
+      const b = Math.floor(Math.sin(i * 0.08 + row + 4) * 127 + 128);
+      content += `${ESC}[48;2;${r};${g};${b}m ${ESC}[0m`;
+    }
+    pushMarkedLine(content);
+  }
+  pushBlankLine();
+
+  pushMarkedLine(`${ESC}[1mCombined styles${ESC}[0m`);
+  pushMarkedLine(
+    `${ESC}[1mBold${ESC}[0m  ${ESC}[3mItalic${ESC}[0m  ${ESC}[4mUnderline${ESC}[0m  ${ESC}[7mReverse${ESC}[0m  ${ESC}[9mStrike${ESC}[0m`
+  );
+  pushMarkedLine(
+    `${ESC}[1;31mBold Red${ESC}[0m  ${ESC}[3;32mItalic Green${ESC}[0m  ${ESC}[4;34mUnderline Blue${ESC}[0m  ${ESC}[38;5;201mPalette Pink${ESC}[0m`
+  );
+  pushBlankLine();
+
+  pushMarkedLine(`${ESC}[1mSoft-wrap metadata seed${ESC}[0m`);
+  pushMarkedLine('wrap '.repeat(48));
+  pushBlankLine();
+
+  pushMarkedLine(`${ESC}[1mUnicode box drawing${ESC}[0m`);
+  for (const line of [
+    '┌──────────┬──────────┬──────────┐',
+    '│  Cell A  │  Cell B  │  Cell C  │',
+    '├──────────┼──────────┼──────────┤',
+    '│  Cell D  │  Cell E  │  Cell F  │',
+    '└──────────┴──────────┴──────────┘',
+  ]) {
+    pushMarkedLine(line);
+  }
+  pushBlankLine();
+
+  for (let section = 0; section < 8; section++) {
+    pushMarkedLine(`${ESC}[1mColor grid ${String.fromCharCode(65 + section)}${ESC}[0m`);
+    for (let row = 0; row < 8; row++) {
+      let content = '';
+      for (let i = 0; i < 70; i++) {
+        const idx = (section * 64 + row * 8 + i) % 256;
+        content +=
+          (i + row) % 3 === 0
+            ? `${ESC}[38;2;${(idx * 7) % 256};${(idx * 13) % 256};${(idx * 23) % 256}m*${ESC}[0m`
+            : `${ESC}[38;5;${idx}m*${ESC}[0m`;
+      }
+      pushMarkedLine(content);
+    }
+    pushBlankLine();
+  }
+
+  pushMarkedLine(`${ESC}[1m${'═'.repeat(72)}${ESC}[0m`);
+  pushMarkedLine(`${ESC}[32m✓${ESC}[0m Test complete`);
+  pushBlankLine();
+
+  const text = lines.join('\r\n') + '\r\n';
+  expect(text.match(/\x1b\[0m/g)?.length ?? 0).toBeGreaterThan(100);
+  expect(text).toContain('\r\n\r\n');
+
+  const data = new TextEncoder().encode(text);
+  expect(data.length).toBeGreaterThan(20_000);
+  return data;
+}
+
+function cellsToText(cells: GhosttyCell[]): string {
+  return cells
+    .filter((cell) => cell.width !== 0)
+    .map((cell) => String.fromCodePoint(cell.codepoint > 32 ? cell.codepoint : 32))
+    .join('')
+    .trimEnd();
+}
+
+function getViewportText(term: Terminal): string[] {
+  expect(term.wasmTerm).toBeDefined();
+  const viewport = term.wasmTerm!.getViewport();
+  const rows: string[] = [];
+
+  for (let row = 0; row < term.rows; row++) {
+    const start = row * term.cols;
+    rows.push(cellsToText(viewport.slice(start, start + term.cols)));
+  }
+
+  return rows;
+}
+
+function getLineText(term: Terminal, row: number): string {
+  expect(term.wasmTerm).toBeDefined();
+  const line = term.wasmTerm!.getLine(row);
+  expect(line).not.toBeNull();
+  return cellsToText(line!);
+}
+
+describe('viewport scroll-growth corruption', () => {
+  test('does not expose stale cells after ESC[0m reset-heavy scrolling', async () => {
+    const data = buildStressPayload();
+    const term = await createIsolatedTerminal({ cols: 160, rows: 39, scrollback: 10_000_000 });
+    const container = document.createElement('div');
+    document.body.appendChild(container);
+
+    try {
+      term.open(container);
+      let baseline: string[] | null = null;
+
+      for (let rep = 0; rep < 30; rep++) {
+        term.write(data);
+        term.wasmTerm!.update();
+        const rows = getViewportText(term);
+
+        for (let row = 0; row < term.rows; row++) {
+          const viewportText = rows[row];
+          const lineText = getLineText(term, row);
+          expect(viewportText).toBe(lineText);
+
+          if (viewportText.length === 0) {
+            expect(term.wasmTerm!.isRowWrapped(row)).toBe(false);
+          }
+
+          const markers = viewportText.match(MARKER_PATTERN) ?? [];
+          const uniqueMarkers = new Set(markers);
+          if (uniqueMarkers.size > 1) {
+            throw new Error(
+              `Rep ${rep}, row ${row} contains merged markers: ${[...uniqueMarkers].join(', ')}\n` +
+                `Row content: ${JSON.stringify(viewportText)}`
+            );
+          }
+        }
+
+        if (baseline === null) {
+          baseline = rows;
+          continue;
+        }
+
+        const changedRow = rows.findIndex((rowText, row) => rowText !== baseline![row]);
+        if (changedRow !== -1) {
+          throw new Error(
+            `Rep ${rep}, row ${changedRow} differs from the stable viewport baseline\n` +
+              `Expected: ${JSON.stringify(baseline[changedRow])}\n` +
+              `Received: ${JSON.stringify(rows[changedRow])}`
+          );
+        }
+      }
+    } finally {
+      term.dispose();
+      document.body.removeChild(container);
+    }
+  });
+});

patches/ghostty-wasm-api.patch

@@ -1564,6 +1564,35 @@ index 000000000..73ae2e6fa
 +    try std.testing.expectEqual(@as(u32, 'l'), cells[3].codepoint);
 +    try std.testing.expectEqual(@as(u32, 'o'), cells[4].codepoint);
 +}
+diff --git a/src/terminal/Screen.zig b/src/terminal/Screen.zig
+index ba2af2473..b8be8f273 100644
+--- a/src/terminal/Screen.zig
++++ b/src/terminal/Screen.zig
+@@ -851,10 +851,19 @@ pub fn cursorDownScroll(self: *Screen) !void {
+-        // Clear the new row so it gets our bg color. We only do this
+-        // if we have a bg color at all.
+-        if (self.cursor.style.bg_color != .none) {
++        // Always clear the new row's cells. When pages.grow() extends an
++        // existing page, the new row's cell memory may contain stale data
++        // from previously erased rows. Without clearing, these stale cells
++        // become visible when the row isn't fully overwritten, such as after
++        // bare CRLF output with the default cursor style.
++        {
+             const page: *Page = &page_pin.node.data;
++            const row = self.cursor.page_row;
++            const cells_offset = row.cells;
++            const dirty = row.dirty;
+             self.clearCells(
+                 page,
+-                self.cursor.page_row,
+-                page.getCells(self.cursor.page_row),
++                row,
++                page.getCells(row),
+             );
++            // Reset row-level metadata that clearCells does not touch while
++            // preserving the backing cell offset and dirty state.
++            row.* = .{ .cells = cells_offset, .dirty = dirty };
+         }
 diff --git a/src/terminal/render.zig b/src/terminal/render.zig
 index b6430ea34..10e0ef79d 100644
 --- a/src/terminal/render.zig