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

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

packages/cli/src/ui/AppContainer.test.tsx

@@ -2675,6 +2675,10 @@ describe('AppContainer State Management', () => {
       isAlternateMode = false,
       childHandler?: Mock,
     ) => {
+      vi.spyOn(mockConfig, 'getUseAlternateBuffer').mockReturnValue(
+        isAlternateMode,
+      );
+
       // Update settings for this test run
       const defaultMergedSettings = mergeSettings({}, {}, {}, {}, true);
       const testSettings = {
@@ -3364,6 +3368,8 @@ describe('AppContainer State Management', () => {
       );
       vi.mocked(checkPermissions).mockResolvedValue([]);
 
+      vi.spyOn(mockConfig, 'getUseAlternateBuffer').mockReturnValue(true);
+
       let unmount: () => void;
       await act(async () => {
         unmount = renderAppContainer({
@@ -3596,6 +3602,8 @@ describe('AppContainer State Management', () => {
         },
       } as unknown as LoadedSettings;
 
+      vi.spyOn(mockConfig, 'getUseAlternateBuffer').mockReturnValue(true);
+
       let unmount: () => void;
       await act(async () => {
         const result = renderAppContainer({

packages/cli/src/ui/AppContainer.tsx

@@ -145,7 +145,6 @@ import { useSessionResume } from './hooks/useSessionResume.js';
 import { useIncludeDirsTrust } from './hooks/useIncludeDirsTrust.js';
 import { useSessionRetentionCheck } from './hooks/useSessionRetentionCheck.js';
 import { isWorkspaceTrusted } from '../config/trustedFolders.js';
-import { useAlternateBuffer } from './hooks/useAlternateBuffer.js';
 import { useSettings } from './contexts/SettingsContext.js';
 import { terminalCapabilityManager } from './utils/terminalCapabilityManager.js';
 import { useInputHistoryStore } from './hooks/useInputHistoryStore.js';
@@ -228,7 +227,7 @@ export const AppContainer = (props: AppContainerProps) => {
   });
 
   useMemoryMonitor(historyManager);
-  const isAlternateBuffer = useAlternateBuffer();
+  const isAlternateBuffer = config.getUseAlternateBuffer();
   const [corgiMode, setCorgiMode] = useState(false);
   const [forceRerenderKey, setForceRerenderKey] = useState(0);
   const [debugMessage, setDebugMessage] = useState<string>('');
@@ -545,7 +544,7 @@ export const AppContainer = (props: AppContainerProps) => {
   const { consoleMessages, clearConsoleMessages: clearConsoleMessagesState } =
     useConsoleMessages();
 
-  const mainAreaWidth = calculateMainAreaWidth(terminalWidth, settings);
+  const mainAreaWidth = calculateMainAreaWidth(terminalWidth, config);
   // Derive widths for InputPrompt using shared helper
   const { inputWidth, suggestionsWidth } = useMemo(() => {
     const { inputWidth, suggestionsWidth } =

packages/cli/src/ui/components/ExitPlanModeDialog.test.tsx

@@ -167,6 +167,7 @@ Implement a comprehensive authentication system with multiple providers.
             readTextFile: vi.fn(),
             writeTextFile: vi.fn(),
           }),
+          getUseAlternateBuffer: () => options?.useAlternateBuffer ?? true,
         } as unknown as import('@google/gemini-cli-core').Config,
       },
     );
@@ -443,6 +444,7 @@ Implement a comprehensive authentication system with multiple providers.
                 readTextFile: vi.fn(),
                 writeTextFile: vi.fn(),
               }),
+              getUseAlternateBuffer: () => useAlternateBuffer ?? true,
             } as unknown as import('@google/gemini-cli-core').Config,
           },
         );

packages/cli/src/ui/components/ToolConfirmationQueue.test.tsx

@@ -51,6 +51,7 @@ describe('ToolConfirmationQueue', () => {
     storage: {
       getPlansDir: () => '/mock/temp/plans',
     },
+    getUseAlternateBuffer: () => false,
   } as unknown as Config;
 
   beforeEach(() => {

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);
 };

packages/cli/src/ui/utils/ui-sizing.test.ts

@@ -4,29 +4,11 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { describe, it, expect, vi } from 'vitest';
+import { describe, it, expect } from 'vitest';
 import { calculateMainAreaWidth } from './ui-sizing.js';
-import { type LoadedSettings } from '../../config/settings.js';
-
-// Mock dependencies
-const mocks = vi.hoisted(() => ({
-  isAlternateBufferEnabled: vi.fn(),
-}));
-
-vi.mock('../hooks/useAlternateBuffer.js', () => ({
-  isAlternateBufferEnabled: mocks.isAlternateBufferEnabled,
-}));
+import type { Config } from '@google/gemini-cli-core';
 
 describe('ui-sizing', () => {
-  const createSettings = (useFullWidth?: boolean): LoadedSettings =>
-    ({
-      merged: {
-        ui: {
-          useFullWidth,
-        },
-      },
-    }) as unknown as LoadedSettings;
-
   describe('calculateMainAreaWidth', () => {
     it.each([
       // expected, width, altBuffer
@@ -37,10 +19,10 @@ describe('ui-sizing', () => {
     ])(
       'should return %i when width=%i and altBuffer=%s',
       (expected, width, altBuffer) => {
-        mocks.isAlternateBufferEnabled.mockReturnValue(altBuffer);
-        const settings = createSettings();
-
-        expect(calculateMainAreaWidth(width, settings)).toBe(expected);
+        const mockConfig = {
+          getUseAlternateBuffer: () => altBuffer,
+        } as unknown as Config;
+        expect(calculateMainAreaWidth(width, mockConfig)).toBe(expected);
       },
     );
   });

packages/cli/src/ui/utils/ui-sizing.ts

@@ -4,14 +4,14 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { type LoadedSettings } from '../../config/settings.js';
+import type { Config } from '@google/gemini-cli-core';
 import { isAlternateBufferEnabled } from '../hooks/useAlternateBuffer.js';
 
 export const calculateMainAreaWidth = (
   terminalWidth: number,
-  settings: LoadedSettings,
+  config: Config,
 ): number => {
-  if (isAlternateBufferEnabled(settings)) {
+  if (isAlternateBufferEnabled(config)) {
     return terminalWidth - 1;
   }
   return terminalWidth;