fix(cli): reduce terminal redraw cursor movement (#3381)

* fix(cli): reduce terminal redraw cursor movement

  Collapse Ink multiline erase sequences into a single relative cursor move plus erase-down operation.

  This avoids excessive repeated cursor-up writes during streaming interactive renders while preserving normal TTY behavior. Screen
  reader mode and non-TTY output are left unchanged, with a legacy env fallback available.

* Optimize Ink multiline erase sequences during interactive TTY rendering.

  Collapse repeated cursor-up movement while preserving bounded line clearing, so redraws avoid excessive upward cursor jumps without erasing
  unrelated terminal output below the frame. Non-TTY output, screen reader mode, and non-string writes are unchanged.

packages/cli/src/gemini.tsx

@@ -70,6 +70,7 @@ import { DualOutputBridge } from './dualOutput/DualOutputBridge.js';
 import { DualOutputContext } from './dualOutput/DualOutputContext.js';
 import { RemoteInputWatcher } from './remoteInput/RemoteInputWatcher.js';
 import { RemoteInputContext } from './remoteInput/RemoteInputContext.js';
+import { installTerminalRedrawOptimizer } from './ui/utils/terminalRedrawOptimizer.js';
 
 const debugLogger = createDebugLogger('STARTUP');
 
@@ -155,6 +156,10 @@ export async function startInteractiveUI(
 ) {
   const version = await getCliVersion();
   setWindowTitle(basename(workspaceRoot), settings);
+  const restoreTerminalRedrawOptimizer =
+    process.stdout.isTTY && !config.getScreenReader()
+      ? installTerminalRedrawOptimizer(process.stdout)
+      : () => {};
 
   // Create dual output bridge if --json-fd or --json-file is specified.
   // Errors are caught so a bad fd/path degrades gracefully instead of
@@ -268,6 +273,7 @@ export async function startInteractiveUI(
     remoteInputWatcher?.shutdown();
     dualOutputBridge?.shutdown();
     instance.unmount();
+    restoreTerminalRedrawOptimizer();
   });
 }
 

packages/cli/src/ui/utils/terminalRedrawOptimizer.test.ts

@@ -0,0 +1,100 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import {
+  installTerminalRedrawOptimizer,
+  optimizeMultilineEraseLines,
+} from './terminalRedrawOptimizer.js';
+
+const ESC = '\u001B[';
+const ERASE_LINE = `${ESC}2K`;
+const CURSOR_UP_ONE = `${ESC}1A`;
+const CURSOR_DOWN_ONE = `${ESC}1B`;
+const CURSOR_LEFT = `${ESC}G`;
+
+describe('optimizeMultilineEraseLines', () => {
+  it('collapses repeated cursor-up movement without erasing below', () => {
+    const input = `${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_LEFT}next frame`;
+
+    expect(optimizeMultilineEraseLines(input)).toBe(
+      `${ESC}2A${ERASE_LINE}${CURSOR_DOWN_ONE}${ERASE_LINE}${CURSOR_DOWN_ONE}${ERASE_LINE}${ESC}2A${CURSOR_LEFT}next frame`,
+    );
+  });
+
+  it('leaves two-line erase sequences unchanged', () => {
+    const input = `${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_LEFT}next frame`;
+
+    expect(optimizeMultilineEraseLines(input)).toBe(input);
+  });
+
+  it('leaves single-line erase sequences unchanged', () => {
+    const input = `${ERASE_LINE}${CURSOR_LEFT}next frame`;
+
+    expect(optimizeMultilineEraseLines(input)).toBe(input);
+  });
+
+  it('optimizes each multiline erase sequence in a chunk', () => {
+    const first = `${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_LEFT}`;
+    const second = `${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_LEFT}`;
+
+    expect(optimizeMultilineEraseLines(`${first}a${second}b`)).toBe(
+      `${first}a${ESC}2A${ERASE_LINE}${CURSOR_DOWN_ONE}${ERASE_LINE}${CURSOR_DOWN_ONE}${ERASE_LINE}${ESC}2A${CURSOR_LEFT}b`,
+    );
+  });
+
+  it('does not emit erase-down sequences', () => {
+    const input = `${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_LEFT}`;
+
+    expect(optimizeMultilineEraseLines(input)).not.toContain(`${ESC}J`);
+  });
+});
+
+describe('installTerminalRedrawOptimizer', () => {
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
+  it('optimizes string writes and restores the original writer', () => {
+    const write = vi.fn(() => true);
+    const stdout = { write } as unknown as NodeJS.WriteStream;
+    const restore = installTerminalRedrawOptimizer(stdout);
+    const input = `${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_UP_ONE}${ERASE_LINE}${CURSOR_LEFT}`;
+
+    stdout.write(input);
+
+    expect(write).toHaveBeenCalledWith(
+      `${ESC}2A${ERASE_LINE}${CURSOR_DOWN_ONE}${ERASE_LINE}${CURSOR_DOWN_ONE}${ERASE_LINE}${ESC}2A${CURSOR_LEFT}`,
+      undefined,
+      undefined,
+    );
+
+    restore();
+    expect(stdout.write).toBe(write);
+  });
+
+  it('passes non-string writes through unchanged', () => {
+    const write = vi.fn(() => true);
+    const stdout = { write } as unknown as NodeJS.WriteStream;
+    installTerminalRedrawOptimizer(stdout);
+    const input = Buffer.from('hello');
+
+    stdout.write(input);
+
+    expect(write).toHaveBeenCalledWith(input, undefined, undefined);
+  });
+
+  it('can be disabled for terminal compatibility fallback', () => {
+    vi.stubEnv('QWEN_CODE_LEGACY_ERASE_LINES', '1');
+    const write = vi.fn(() => true);
+    const stdout = { write } as unknown as NodeJS.WriteStream;
+    const restore = installTerminalRedrawOptimizer(stdout);
+
+    expect(stdout.write).toBe(write);
+    restore();
+    expect(stdout.write).toBe(write);
+  });
+});

packages/cli/src/ui/utils/terminalRedrawOptimizer.ts

@@ -0,0 +1,98 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+const ESC = '\u001B[';
+const ERASE_LINE = `${ESC}2K`;
+const CURSOR_UP_ONE = `${ESC}1A`;
+const CURSOR_DOWN_ONE = `${ESC}1B`;
+const CURSOR_LEFT = `${ESC}G`;
+
+const MULTILINE_ERASE_LINES_PATTERN = new RegExp(
+  `(?:${escapeRegExp(ERASE_LINE + CURSOR_UP_ONE)})+${escapeRegExp(
+    ERASE_LINE + CURSOR_LEFT,
+  )}`,
+  'g',
+);
+
+function escapeRegExp(value: string): string {
+  return value.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+}
+
+function countOccurrences(value: string, search: string): number {
+  let count = 0;
+  let index = 0;
+
+  while ((index = value.indexOf(search, index)) !== -1) {
+    count++;
+    index += search.length;
+  }
+
+  return count;
+}
+
+/**
+ * Ink clears dynamic output via ansi-escapes.eraseLines(), which emits a
+ * clear-line + cursor-up pair for every previous line. That can make terminal
+ * scrollback bounce during frequent streaming renders. Collapse the repeated
+ * upward cursor movement while still clearing only the same old frame lines.
+ */
+export function optimizeMultilineEraseLines(output: string): string {
+  return output.replace(MULTILINE_ERASE_LINES_PATTERN, (sequence) => {
+    const lineCount = countOccurrences(sequence, ERASE_LINE);
+    const cursorUpCount = lineCount - 1;
+
+    if (cursorUpCount <= 1) {
+      return sequence;
+    }
+
+    let boundedErase = `${ESC}${cursorUpCount}A`;
+
+    for (let line = 0; line < lineCount; line++) {
+      boundedErase += ERASE_LINE;
+
+      if (line < lineCount - 1) {
+        boundedErase += CURSOR_DOWN_ONE;
+      }
+    }
+
+    return `${boundedErase}${ESC}${cursorUpCount}A${CURSOR_LEFT}`;
+  });
+}
+
+export function installTerminalRedrawOptimizer(
+  stdout: NodeJS.WriteStream,
+): () => void {
+  if (process.env['QWEN_CODE_LEGACY_ERASE_LINES'] === '1') {
+    return () => {};
+  }
+
+  const originalWrite = stdout.write;
+
+  const optimizedWrite = function (
+    this: NodeJS.WriteStream,
+    chunk: unknown,
+    encodingOrCallback?: BufferEncoding | ((error?: Error | null) => void),
+    callback?: (error?: Error | null) => void,
+  ) {
+    const optimizedChunk =
+      typeof chunk === 'string' ? optimizeMultilineEraseLines(chunk) : chunk;
+
+    return originalWrite.call(
+      this,
+      optimizedChunk as string | Uint8Array,
+      encodingOrCallback as BufferEncoding,
+      callback,
+    );
+  } as typeof stdout.write;
+
+  stdout.write = optimizedWrite;
+
+  return () => {
+    if (stdout.write === optimizedWrite) {
+      stdout.write = originalWrite;
+    }
+  };
+}