fix(ui): fix flickering on small terminal heights

packages/cli/src/ui/components/AnsiOutput.tsx

@@ -35,7 +35,11 @@ export const AnsiOutputText: React.FC<AnsiOutputProps> = ({
       ? Math.min(availableHeightLimit, maxLines)
       : (availableHeightLimit ?? maxLines ?? DEFAULT_HEIGHT);
 
-  const lastLines = disableTruncation ? data : data.slice(-numLinesRetained);
+  const lastLines = disableTruncation
+    ? data
+    : numLinesRetained === 0
+      ? []
+      : data.slice(-numLinesRetained);
   return (
     <Box flexDirection="column" width={width} flexShrink={0} overflow="hidden">
       {lastLines.map((line: AnsiLine, lineIndex: number) => (

packages/cli/src/ui/components/MainContent.tsx

@@ -48,6 +48,7 @@ export const MainContent = () => {
     pendingHistoryItems,
     mainAreaWidth,
     staticAreaMaxItemHeight,
+    availableTerminalHeight,
     cleanUiDetailsVisible,
   } = uiState;
   const showHeaderDetails = cleanUiDetailsVisible;
@@ -141,7 +142,7 @@ export const MainContent = () => {
             <HistoryItemDisplay
               key={i}
               availableTerminalHeight={
-                uiState.constrainHeight ? staticAreaMaxItemHeight : undefined
+                uiState.constrainHeight ? availableTerminalHeight : undefined
               }
               terminalWidth={mainAreaWidth}
               item={{ ...item, id: 0 }}
@@ -160,7 +161,7 @@ export const MainContent = () => {
     [
       pendingHistoryItems,
       uiState.constrainHeight,
-      staticAreaMaxItemHeight,
+      availableTerminalHeight,
       mainAreaWidth,
       showConfirmationQueue,
       confirmingTool,

packages/cli/src/ui/components/__snapshots__/MainContent.test.tsx.snap

@@ -6,11 +6,12 @@ AppHeader(full)
 ╭──────────────────────────────────────────────────────────────────────────────────────────────╮
 │ ⊶  Shell Command Running a long command...                                                   │
 │                                                                                              │
+│ Line 9                                                                                       │
 │ Line 10                                                                                      │
 │ Line 11                                                                                      │
 │ Line 12                                                                                      │
 │ Line 13                                                                                      │
-│ Line 14                                                                                      │
+│ Line 14                                                                                    █ │
 │ Line 15                                                                                    █ │
 │ Line 16                                                                                    █ │
 │ Line 17                                                                                    █ │
@@ -27,11 +28,12 @@ AppHeader(full)
 ╭──────────────────────────────────────────────────────────────────────────────────────────────╮
 │ ⊶  Shell Command Running a long command...                                                   │
 │                                                                                              │
+│ Line 9                                                                                       │
 │ Line 10                                                                                      │
 │ Line 11                                                                                      │
 │ Line 12                                                                                      │
 │ Line 13                                                                                      │
-│ Line 14                                                                                      │
+│ Line 14                                                                                    █ │
 │ Line 15                                                                                    █ │
 │ Line 16                                                                                    █ │
 │ Line 17                                                                                    █ │
@@ -47,7 +49,9 @@ exports[`MainContent > MainContent Tool Output Height Logic > 'Normal mode - Con
 ╭──────────────────────────────────────────────────────────────────────────────────────────────╮
 │ ⊶  Shell Command Running a long command...                                                   │
 │                                                                                              │
-│ ... first 11 lines hidden (Ctrl+O to show) ...                                               │
+│ ... first 9 lines hidden (Ctrl+O to show) ...                                                │
+│ Line 10                                                                                      │
+│ Line 11                                                                                      │
 │ Line 12                                                                                      │
 │ Line 13                                                                                      │
 │ Line 14                                                                                      │

packages/cli/src/ui/components/messages/ShellToolMessage.test.tsx

@@ -203,7 +203,7 @@ describe('<ShellToolMessage />', () => {
       [
         'uses full availableTerminalHeight when focused in alternate buffer mode',
         100,
-        98, // 100 - 2
+        98,
         true,
         false,
       ],

packages/cli/src/ui/components/shared/SlicingMaxSizedBox.tsx

@@ -46,7 +46,7 @@ export function SlicingMaxSizedBox<T>({
           text = '...' + text.slice(-MAXIMUM_RESULT_DISPLAY_CHARACTERS);
         }
       }
-      if (maxLines) {
+      if (maxLines !== undefined) {
         const hasTrailingNewline = text.endsWith('\n');
         const contentText = hasTrailingNewline ? text.slice(0, -1) : text;
         const lines = contentText.split('\n');
@@ -71,7 +71,7 @@ export function SlicingMaxSizedBox<T>({
       };
     }
 
-    if (Array.isArray(data) && !isAlternateBuffer && maxLines) {
+    if (Array.isArray(data) && !isAlternateBuffer && maxLines !== undefined) {
       if (data.length > maxLines) {
         // We will have a label from MaxSizedBox. Reserve space for it.
         const targetLines = Math.max(1, maxLines - 1);

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

@@ -0,0 +1,208 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import {
+  calculateToolContentMaxLines,
+  calculateShellMaxLines,
+  SHELL_CONTENT_OVERHEAD,
+} from './toolLayoutUtils.js';
+import { CoreToolCallStatus } from '@google/gemini-cli-core';
+import {
+  ACTIVE_SHELL_MAX_LINES,
+  COMPLETED_SHELL_MAX_LINES,
+} from '../constants.js';
+
+describe('toolLayoutUtils', () => {
+  describe('calculateToolContentMaxLines', () => {
+    interface CalculateToolContentMaxLinesTestCase {
+      desc: string;
+      options: Parameters<typeof calculateToolContentMaxLines>[0];
+      expected: number | undefined;
+    }
+
+    const testCases: CalculateToolContentMaxLinesTestCase[] = [
+      {
+        desc: 'returns undefined if availableTerminalHeight is undefined',
+        options: {
+          availableTerminalHeight: undefined,
+          isAlternateBuffer: false,
+        },
+        expected: undefined,
+      },
+      {
+        desc: 'returns maxLinesLimit if maxLinesLimit applies but availableTerminalHeight is undefined',
+        options: {
+          availableTerminalHeight: undefined,
+          isAlternateBuffer: false,
+          maxLinesLimit: 10,
+        },
+        expected: 10,
+      },
+      {
+        desc: 'returns available space directly in constrained terminal (Standard mode)',
+        options: {
+          availableTerminalHeight: 2,
+          isAlternateBuffer: false,
+        },
+        expected: 3,
+      },
+      {
+        desc: 'returns available space directly in constrained terminal (ASB mode)',
+        options: {
+          availableTerminalHeight: 4,
+          isAlternateBuffer: true,
+        },
+        expected: 3,
+      },
+      {
+        desc: 'returns remaining space if sufficient space exists (Standard mode)',
+        options: {
+          availableTerminalHeight: 20,
+          isAlternateBuffer: false,
+        },
+        expected: 17,
+      },
+      {
+        desc: 'returns remaining space if sufficient space exists (ASB mode)',
+        options: {
+          availableTerminalHeight: 20,
+          isAlternateBuffer: true,
+        },
+        expected: 13,
+      },
+    ];
+
+    it.each(testCases)('$desc', ({ options, expected }) => {
+      const result = calculateToolContentMaxLines(options);
+      expect(result).toBe(expected);
+    });
+  });
+
+  describe('calculateShellMaxLines', () => {
+    interface CalculateShellMaxLinesTestCase {
+      desc: string;
+      options: Parameters<typeof calculateShellMaxLines>[0];
+      expected: number | undefined;
+    }
+
+    const testCases: CalculateShellMaxLinesTestCase[] = [
+      {
+        desc: 'returns undefined when not constrained and is expandable',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: false,
+          isThisShellFocused: false,
+          availableTerminalHeight: 20,
+          constrainHeight: false,
+          isExpandable: true,
+        },
+        expected: undefined,
+      },
+      {
+        desc: 'returns ACTIVE_SHELL_MAX_LINES - SHELL_CONTENT_OVERHEAD for ASB mode when availableTerminalHeight is undefined',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: true,
+          isThisShellFocused: false,
+          availableTerminalHeight: undefined,
+          constrainHeight: true,
+          isExpandable: false,
+        },
+        expected: ACTIVE_SHELL_MAX_LINES - SHELL_CONTENT_OVERHEAD,
+      },
+      {
+        desc: 'returns undefined for Standard mode when availableTerminalHeight is undefined',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: false,
+          isThisShellFocused: false,
+          availableTerminalHeight: undefined,
+          constrainHeight: true,
+          isExpandable: false,
+        },
+        expected: undefined,
+      },
+      {
+        desc: 'handles small availableTerminalHeight gracefully without overflow in Standard mode',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: false,
+          isThisShellFocused: false,
+          availableTerminalHeight: 2,
+          constrainHeight: true,
+          isExpandable: false,
+        },
+        expected: 1,
+      },
+      {
+        desc: 'handles small availableTerminalHeight gracefully without overflow in ASB mode',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: true,
+          isThisShellFocused: false,
+          availableTerminalHeight: 6,
+          constrainHeight: true,
+          isExpandable: false,
+        },
+        expected: 4,
+      },
+      {
+        desc: 'handles negative availableTerminalHeight gracefully',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: false,
+          isThisShellFocused: false,
+          availableTerminalHeight: -5,
+          constrainHeight: true,
+          isExpandable: false,
+        },
+        expected: 1,
+      },
+      {
+        desc: 'returns maxLinesBasedOnHeight for focused ASB shells',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: true,
+          isThisShellFocused: true,
+          availableTerminalHeight: 30,
+          constrainHeight: false,
+          isExpandable: false,
+        },
+        expected: 28,
+      },
+      {
+        desc: 'falls back to COMPLETED_SHELL_MAX_LINES - SHELL_CONTENT_OVERHEAD for completed shells if space allows',
+        options: {
+          status: CoreToolCallStatus.Success,
+          isAlternateBuffer: false,
+          isThisShellFocused: false,
+          availableTerminalHeight: 100,
+          constrainHeight: true,
+          isExpandable: false,
+        },
+        expected: COMPLETED_SHELL_MAX_LINES - SHELL_CONTENT_OVERHEAD,
+      },
+      {
+        desc: 'falls back to ACTIVE_SHELL_MAX_LINES - SHELL_CONTENT_OVERHEAD for executing shells if space allows',
+        options: {
+          status: CoreToolCallStatus.Executing,
+          isAlternateBuffer: false,
+          isThisShellFocused: false,
+          availableTerminalHeight: 100,
+          constrainHeight: true,
+          isExpandable: false,
+        },
+        expected: ACTIVE_SHELL_MAX_LINES - SHELL_CONTENT_OVERHEAD,
+      },
+    ];
+
+    it.each(testCases)('$desc', ({ options, expected }) => {
+      const result = calculateShellMaxLines(options);
+      expect(result).toBe(expected);
+    });
+  });
+});

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

@@ -46,12 +46,13 @@ export function calculateToolContentMaxLines(options: {
     ? TOOL_RESULT_ASB_RESERVED_LINE_COUNT
     : TOOL_RESULT_STANDARD_RESERVED_LINE_COUNT;
 
-  let contentHeight = availableTerminalHeight
-    ? Math.max(
-        availableTerminalHeight - TOOL_RESULT_STATIC_HEIGHT - reservedLines,
-        TOOL_RESULT_MIN_LINES_SHOWN + 1,
-      )
-    : undefined;
+  let contentHeight =
+    availableTerminalHeight !== undefined
+      ? Math.max(
+          availableTerminalHeight - TOOL_RESULT_STATIC_HEIGHT - reservedLines,
+          TOOL_RESULT_MIN_LINES_SHOWN + 1,
+        )
+      : undefined;
 
   if (maxLinesLimit !== undefined) {
     contentHeight =
@@ -100,7 +101,10 @@ export function calculateShellMaxLines(options: {
       : undefined;
   }
 
-  const maxLinesBasedOnHeight = Math.max(1, availableTerminalHeight - 2);
+  const maxLinesBasedOnHeight = Math.max(
+    1,
+    availableTerminalHeight - TOOL_RESULT_STANDARD_RESERVED_LINE_COUNT,
+  );
 
   // 3. Handle ASB mode focus expansion.
   // We allow a focused shell in ASB mode to take up the full available height,