refactor(core,cli): useAlternateBuffer read from config (#20346)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/cli/src/ui/hooks/useAlternateBuffer.test.ts

@@ -0,0 +1,80 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { renderHook } from '../../test-utils/render.js';
+import {
+  useAlternateBuffer,
+  isAlternateBufferEnabled,
+} from './useAlternateBuffer.js';
+import type { Config } from '@google/gemini-cli-core';
+
+vi.mock('../contexts/ConfigContext.js', () => ({
+  useConfig: vi.fn(),
+}));
+
+const mockUseConfig = vi.mocked(
+  await import('../contexts/ConfigContext.js').then((m) => m.useConfig),
+);
+
+describe('useAlternateBuffer', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('should return false when config.getUseAlternateBuffer returns false', () => {
+    mockUseConfig.mockReturnValue({
+      getUseAlternateBuffer: () => false,
+    } as unknown as ReturnType<typeof mockUseConfig>);
+
+    const { result } = renderHook(() => useAlternateBuffer());
+    expect(result.current).toBe(false);
+  });
+
+  it('should return true when config.getUseAlternateBuffer returns true', () => {
+    mockUseConfig.mockReturnValue({
+      getUseAlternateBuffer: () => true,
+    } as unknown as ReturnType<typeof mockUseConfig>);
+
+    const { result } = renderHook(() => useAlternateBuffer());
+    expect(result.current).toBe(true);
+  });
+
+  it('should return the immutable config value, not react to settings changes', () => {
+    const mockConfig = {
+      getUseAlternateBuffer: () => true,
+    } as unknown as ReturnType<typeof mockUseConfig>;
+
+    mockUseConfig.mockReturnValue(mockConfig);
+
+    const { result, rerender } = renderHook(() => useAlternateBuffer());
+
+    // Value should remain true even after rerender
+    expect(result.current).toBe(true);
+
+    rerender();
+
+    expect(result.current).toBe(true);
+  });
+});
+
+describe('isAlternateBufferEnabled', () => {
+  it('should return true when config.getUseAlternateBuffer returns true', () => {
+    const config = {
+      getUseAlternateBuffer: () => true,
+    } as unknown as Config;
+
+    expect(isAlternateBufferEnabled(config)).toBe(true);
+  });
+
+  it('should return false when config.getUseAlternateBuffer returns false', () => {
+    const config = {
+      getUseAlternateBuffer: () => false,
+    } as unknown as Config;
+
+    expect(isAlternateBufferEnabled(config)).toBe(false);
+  });
+});

packages/cli/src/ui/hooks/useAlternateBuffer.ts

@@ -4,13 +4,14 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { useSettings } from '../contexts/SettingsContext.js';
-import type { LoadedSettings } from '../../config/settings.js';
+import { useConfig } from '../contexts/ConfigContext.js';
+import type { Config } from '@google/gemini-cli-core';
 
-export const isAlternateBufferEnabled = (settings: LoadedSettings): boolean =>
-  settings.merged.ui.useAlternateBuffer === true;
+export const isAlternateBufferEnabled = (config: Config): boolean =>
+  config.getUseAlternateBuffer();
 
+// This is read from Config so that the UI reads the same value per application session
 export const useAlternateBuffer = (): boolean => {
-  const settings = useSettings();
-  return isAlternateBufferEnabled(settings);
+  const config = useConfig();
+  return isAlternateBufferEnabled(config);
 };