feat(ui) Make useAlternateBuffer the default (#12976)

packages/cli/src/ui/utils/CodeColorizer.test.tsx

@@ -0,0 +1,49 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import { colorizeCode } from './CodeColorizer.js';
+import { renderWithProviders } from '../../test-utils/render.js';
+import { LoadedSettings } from '../../config/settings.js';
+
+describe('colorizeCode', () => {
+  it('renders empty lines correctly when useAlternateBuffer is true', () => {
+    const code = 'line 1\n\nline 3';
+    const settings = new LoadedSettings(
+      { path: '', settings: {}, originalSettings: {} },
+      { path: '', settings: {}, originalSettings: {} },
+      {
+        path: '',
+        settings: { ui: { useAlternateBuffer: true, showLineNumbers: false } },
+        originalSettings: {
+          ui: { useAlternateBuffer: true, showLineNumbers: false },
+        },
+      },
+      { path: '', settings: {}, originalSettings: {} },
+      true,
+      new Set(),
+    );
+
+    const result = colorizeCode({
+      code,
+      language: 'javascript',
+      maxWidth: 80,
+      settings,
+      hideLineNumbers: true,
+    });
+
+    const { lastFrame } = renderWithProviders(<>{result}</>);
+    // We expect the output to preserve the empty line.
+    // If the bug exists, it might look like "line 1\nline 3"
+    // If fixed, it should look like "line 1\n \nline 3" (if we use space) or just have the newline.
+
+    // We can check if the output matches the code (ignoring color codes if any, but lastFrame returns plain text usually unless configured otherwise)
+    // Actually lastFrame() returns string with ANSI codes stripped by default in some setups, or not.
+    // But ink-testing-library usually returns the visual representation.
+
+    expect(lastFrame()).toMatch(/line 1\s*\n\s*\n\s*line 3/);
+  });
+});

packages/cli/src/ui/utils/CodeColorizer.tsx

@@ -22,6 +22,7 @@ import {
 } from '../components/shared/MaxSizedBox.js';
 import type { LoadedSettings } from '../../config/settings.js';
 import { debugLogger } from '@google/gemini-cli-core';
+import { isAlternateBufferEnabled } from '../hooks/useAlternateBuffer.js';
 
 // Configure theming and parsing utilities.
 const lowlight = createLowlight(common);
@@ -150,7 +151,7 @@ export function colorizeCode({
     ? false
     : (settings?.merged.ui?.showLineNumbers ?? true);
 
-  const useMaxSizedBox = settings?.merged.ui?.useAlternateBuffer !== true;
+  const useMaxSizedBox = !isAlternateBufferEnabled(settings);
   try {
     // Render the HAST tree using the adapted theme
     // Apply the theme's default foreground color to the top-level Text element
@@ -160,10 +161,7 @@ export function colorizeCode({
     let hiddenLinesCount = 0;
 
     // Optimization to avoid highlighting lines that cannot possibly be displayed.
-    if (
-      availableHeight !== undefined &&
-      settings?.merged.ui?.useAlternateBuffer === false
-    ) {
+    if (availableHeight !== undefined && useMaxSizedBox) {
       availableHeight = Math.max(availableHeight, MINIMUM_MAX_HEIGHT);
       if (lines.length > availableHeight) {
         const sliceIndex = lines.length - availableHeight;
@@ -180,7 +178,7 @@ export function colorizeCode({
       );
 
       return (
-        <Box key={index}>
+        <Box key={index} minHeight={useMaxSizedBox ? undefined : 1}>
           {/* We have to render line numbers differently depending on whether we are using MaxSizeBox or not */}
           {showLineNumbers && useMaxSizedBox && (
             <Text color={activeTheme.colors.Gray}>
@@ -238,7 +236,7 @@ export function colorizeCode({
     const lines = codeToHighlight.split('\n');
     const padWidth = String(lines.length).length; // Calculate padding width based on number of lines
     const fallbackLines = lines.map((line, index) => (
-      <Box key={index}>
+      <Box key={index} minHeight={useMaxSizedBox ? undefined : 1}>
         {/* We have to render line numbers differently depending on whether we are using MaxSizeBox or not */}
         {showLineNumbers && useMaxSizedBox && (
           <Text color={activeTheme.defaultColor}>

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

@@ -6,6 +6,7 @@
 
 import { lerp } from '../../utils/math.js';
 import { type LoadedSettings } from '../../config/settings.js';
+import { isAlternateBufferEnabled } from '../hooks/useAlternateBuffer.js';
 
 const getMainAreaWidthInternal = (terminalWidth: number): number => {
   if (terminalWidth <= 80) {
@@ -27,7 +28,7 @@ export const calculateMainAreaWidth = (
   settings: LoadedSettings,
 ): number => {
   if (settings.merged.ui?.useFullWidth !== false) {
-    if (settings.merged.ui?.useAlternateBuffer) {
+    if (isAlternateBufferEnabled(settings)) {
       return terminalWidth - 1;
     }
     return terminalWidth;