feat(cli): improve CTRL+O experience for both standard and alternate screen buffer (ASB) modes (#19010)

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

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

@@ -0,0 +1,110 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  ACTIVE_SHELL_MAX_LINES,
+  COMPLETED_SHELL_MAX_LINES,
+} from '../constants.js';
+import { CoreToolCallStatus } from '@google/gemini-cli-core';
+
+/**
+ * Constants used for calculating available height for tool results.
+ * These MUST be kept in sync between ToolGroupMessage (for overflow detection)
+ * and ToolResultDisplay (for actual truncation).
+ */
+export const TOOL_RESULT_STATIC_HEIGHT = 1;
+export const TOOL_RESULT_ASB_RESERVED_LINE_COUNT = 6;
+export const TOOL_RESULT_STANDARD_RESERVED_LINE_COUNT = 2;
+export const TOOL_RESULT_MIN_LINES_SHOWN = 2;
+
+/**
+ * Calculates the final height available for the content of a tool result display.
+ *
+ * This accounts for:
+ * 1. The static height of the tool message (name, status line).
+ * 2. Reserved space for hints and padding (different in ASB vs Standard mode).
+ * 3. Enforcing a minimum number of lines shown.
+ */
+export function calculateToolContentMaxLines(options: {
+  availableTerminalHeight: number | undefined;
+  isAlternateBuffer: boolean;
+  maxLinesLimit?: number;
+}): number | undefined {
+  const { availableTerminalHeight, isAlternateBuffer, maxLinesLimit } = options;
+
+  const reservedLines = isAlternateBuffer
+    ? 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;
+
+  if (maxLinesLimit) {
+    contentHeight =
+      contentHeight !== undefined
+        ? Math.min(contentHeight, maxLinesLimit)
+        : maxLinesLimit;
+  }
+
+  return contentHeight;
+}
+
+/**
+ * Calculates the maximum number of lines to display for shell output.
+ *
+ * This logic distinguishes between:
+ * 1. Process Status: Active (Executing) vs Completed.
+ * 2. UI Focus: Whether the user is currently interacting with the shell.
+ * 3. Expansion State: Whether the user has explicitly requested to "Show More Lines" (CTRL+O).
+ */
+export function calculateShellMaxLines(options: {
+  status: CoreToolCallStatus;
+  isAlternateBuffer: boolean;
+  isThisShellFocused: boolean;
+  availableTerminalHeight: number | undefined;
+  constrainHeight: boolean;
+  isExpandable: boolean | undefined;
+}): number | undefined {
+  const {
+    status,
+    isAlternateBuffer,
+    isThisShellFocused,
+    availableTerminalHeight,
+    constrainHeight,
+    isExpandable,
+  } = options;
+
+  // 1. If the user explicitly requested expansion (unconstrained), remove all caps.
+  if (!constrainHeight && isExpandable) {
+    return undefined;
+  }
+
+  // 2. Handle cases where height is unknown (Standard mode history).
+  if (availableTerminalHeight === undefined) {
+    return isAlternateBuffer ? ACTIVE_SHELL_MAX_LINES : undefined;
+  }
+
+  const maxLinesBasedOnHeight = Math.max(1, availableTerminalHeight - 2);
+
+  // 3. Handle ASB mode focus expansion.
+  // We allow a focused shell in ASB mode to take up the full available height,
+  // BUT only if we aren't trying to maintain a constrained view (e.g., history items).
+  if (isAlternateBuffer && isThisShellFocused && !constrainHeight) {
+    return maxLinesBasedOnHeight;
+  }
+
+  // 4. Fall back to process-based constants.
+  const isExecuting = status === CoreToolCallStatus.Executing;
+  const shellMaxLinesLimit = isExecuting
+    ? ACTIVE_SHELL_MAX_LINES
+    : COMPLETED_SHELL_MAX_LINES;
+
+  return Math.min(maxLinesBasedOnHeight, shellMaxLinesLimit);
+}