test(cli): add unit tests for toggleAgentMessageCollapse in use-chat-messages

Adds 14 meaningful tests for the collapse toggle state transformation logic:
- Expanding collapsed messages (isCollapsed, userOpened)
- Collapsing expanded messages
- Default state handling
- Non-agent message passthrough
- Immutability guarantees
- Toggle cycle behavior

Removed trivial tests for pagination/array slicing per code review feedback.

Author: brandonkachen

cli/src/hooks/__tests__/use-chat-messages.test.ts

@@ -0,0 +1,218 @@
+import { describe, test, expect } from 'bun:test'
+
+import type { ChatMessage } from '../../types/chat'
+
+/**
+ * Tests for useChatMessages hook logic.
+ *
+ * These tests focus on the non-trivial state transformation logic:
+ * - Collapse toggle behavior for agent messages (isCollapsed, userOpened)
+ *
+ * Note: Pagination logic (slice, hidden count, load more) is trivial
+ * JavaScript operations and doesn't warrant unit testing.
+ */
+
+// ============================================================================
+// Extracted pure functions (mirror the hook's logic)
+// ============================================================================
+
+/**
+ * Toggles the collapsed state of an agent message.
+ * Mirrors the handleCollapseToggle logic for agent variant messages.
+ */
+const toggleAgentMessageCollapse = (message: ChatMessage): ChatMessage => {
+  if (message.variant !== 'agent') return message
+
+  const wasCollapsed = message.metadata?.isCollapsed ?? false
+  return {
+    ...message,
+    metadata: {
+      ...message.metadata,
+      isCollapsed: !wasCollapsed,
+      // Mark as user-opened if expanding (wasCollapsed was true)
+      userOpened: wasCollapsed,
+    },
+  }
+}
+
+/**
+ * Creates a minimal agent message for testing.
+ */
+const createAgentMessage = (
+  id: string,
+  options: {
+    isCollapsed?: boolean
+    userOpened?: boolean
+    parentId?: string
+  } = {},
+): ChatMessage => ({
+  id,
+  variant: 'agent',
+  content: '',
+  timestamp: new Date().toISOString(),
+  parentId: options.parentId,
+  metadata: {
+    isCollapsed: options.isCollapsed,
+    userOpened: options.userOpened,
+  },
+})
+
+/**
+ * Creates a minimal user message for testing.
+ */
+const createUserMessage = (
+  id: string,
+  options: { parentId?: string } = {},
+): ChatMessage => ({
+  id,
+  variant: 'user',
+  timestamp: new Date().toISOString(),
+  content: 'test message',
+  parentId: options.parentId,
+})
+
+// ============================================================================
+// Tests
+// ============================================================================
+
+describe('useChatMessages - toggleAgentMessageCollapse', () => {
+  describe('expanding collapsed messages', () => {
+    test('sets isCollapsed to false when was true', () => {
+      const message = createAgentMessage('agent-1', { isCollapsed: true })
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result.metadata?.isCollapsed).toBe(false)
+    })
+
+    test('sets userOpened to true when expanding', () => {
+      const message = createAgentMessage('agent-1', { isCollapsed: true })
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result.metadata?.userOpened).toBe(true)
+    })
+
+    test('preserves other metadata when expanding', () => {
+      const message: ChatMessage = {
+        ...createAgentMessage('agent-1', { isCollapsed: true }),
+        metadata: {
+          isCollapsed: true,
+          customField: 'preserved',
+        } as ChatMessage['metadata'],
+      }
+      const result = toggleAgentMessageCollapse(message)
+
+      expect((result.metadata as Record<string, unknown>)?.customField).toBe('preserved')
+    })
+  })
+
+  describe('collapsing expanded messages', () => {
+    test('sets isCollapsed to true when was false', () => {
+      const message = createAgentMessage('agent-1', { isCollapsed: false })
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result.metadata?.isCollapsed).toBe(true)
+    })
+
+    test('sets userOpened to false when collapsing', () => {
+      const message = createAgentMessage('agent-1', { isCollapsed: false })
+      const result = toggleAgentMessageCollapse(message)
+
+      // When collapsing (wasCollapsed=false), userOpened is set to wasCollapsed (false)
+      expect(result.metadata?.userOpened).toBe(false)
+    })
+  })
+
+  describe('default state handling', () => {
+    test('treats undefined isCollapsed as false (expanded)', () => {
+      const message = createAgentMessage('agent-1') // No isCollapsed set
+      const result = toggleAgentMessageCollapse(message)
+
+      // Should collapse (toggle from default false to true)
+      expect(result.metadata?.isCollapsed).toBe(true)
+      // userOpened should be false (wasCollapsed was false by default)
+      expect(result.metadata?.userOpened).toBe(false)
+    })
+
+    test('handles message with no metadata', () => {
+      const message: ChatMessage = {
+        id: 'agent-1',
+        variant: 'agent',
+        content: '',
+        timestamp: new Date().toISOString(),
+      }
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result.metadata?.isCollapsed).toBe(true)
+    })
+  })
+
+  describe('non-agent messages', () => {
+    test('returns user message unchanged', () => {
+      const message = createUserMessage('user-1')
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result).toBe(message) // Same reference
+    })
+
+    test('does not modify user message metadata', () => {
+      const message = createUserMessage('user-1')
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result.metadata).toEqual(message.metadata)
+    })
+  })
+
+  describe('immutability', () => {
+    test('does not mutate original message', () => {
+      const message = createAgentMessage('agent-1', { isCollapsed: true })
+      const originalCollapsed = message.metadata?.isCollapsed
+
+      toggleAgentMessageCollapse(message)
+
+      expect(message.metadata?.isCollapsed).toBe(originalCollapsed)
+    })
+
+    test('creates new message object', () => {
+      const message = createAgentMessage('agent-1', { isCollapsed: true })
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result).not.toBe(message)
+    })
+
+    test('creates new metadata object', () => {
+      const message = createAgentMessage('agent-1', { isCollapsed: true })
+      const result = toggleAgentMessageCollapse(message)
+
+      expect(result.metadata).not.toBe(message.metadata)
+    })
+  })
+
+  describe('toggle cycle', () => {
+    test('collapse then expand preserves correct state transitions', () => {
+      // Start with expanded message
+      let message = createAgentMessage('agent-1', { isCollapsed: false })
+
+      // Collapse it
+      message = toggleAgentMessageCollapse(message)
+      expect(message.metadata?.isCollapsed).toBe(true)
+      expect(message.metadata?.userOpened).toBe(false)
+
+      // Expand it again
+      message = toggleAgentMessageCollapse(message)
+      expect(message.metadata?.isCollapsed).toBe(false)
+      expect(message.metadata?.userOpened).toBe(true) // Marked as user-opened
+    })
+
+    test('rapid toggle preserves correct state', () => {
+      let message = createAgentMessage('agent-1', { isCollapsed: false })
+
+      // Toggle many times
+      for (let i = 0; i < 10; i++) {
+        message = toggleAgentMessageCollapse(message)
+      }
+
+      // Even number of toggles = back to original state
+      expect(message.metadata?.isCollapsed).toBe(false)
+    })
+  })
+})