refactor(cli): extract multiline input editing handlers (Commit 2.5b)

Extract editing functionality into dedicated hooks:
- use-text-selection.ts: Selection management (getSelectionRange,
  clearSelection, deleteSelection)
- use-text-editing.ts: Character input, cursor movement,
  insertTextAtCursor
- use-mouse-input.ts: Mouse click handling, click-to-cursor positioning

Also extracts TAB_WIDTH constant to shared constants.ts (was
duplicated).

Reduces multiline-input.tsx from ~560 to ~320 lines (-43%).
Total reduction from original: ~1,102 to ~320 lines (-71%).

All 1,911 CLI tests pass. 6 tmux behavior tests pass.
Reviewed by 4 CLI agents (Codebuff, Codex, Claude Code, Gemini).

Author: brandonkachen

REFACTORING_PLAN.md

@@ -15,8 +15,8 @@ This document outlines a prioritized refactoring plan for the 51 issues identifi
 
 ## Progress Tracker
 
-> **Last Updated:** 2025-01-20 (Commits 2.1–2.5a Complete)
-> **Current Status:** Ready for Commit 2.5b (multiline editing handlers)
+> **Last Updated:** 2025-01-20 (Commits 2.1–2.5b Complete)
+> **Current Status:** Ready for Commit 2.6 (use-activity-query)
 
 ### Phase 1 Progress
 | Commit | Description | Status | Completed By |
@@ -35,7 +35,7 @@ This document outlines a prioritized refactoring plan for the 51 issues identifi
 | 2.3 | Refactor loopAgentSteps | ✅ Complete | Codex CLI |
 | 2.4 | Consolidate billing duplication | ✅ Complete | Codex CLI |
 | 2.5a | Extract multiline keyboard navigation | ✅ Complete | Codebuff |
-| 2.5b | Extract multiline editing handlers | ⬜ Not Started | - |
+| 2.5b | Extract multiline editing handlers | ✅ Complete | Codebuff |
 | 2.6 | Simplify use-activity-query.ts | ⬜ Not Started | - |
 | 2.7 | Consolidate XML parsing | ⬜ Not Started | - |
 | 2.8 | Consolidate analytics | ⬜ Not Started | - |
@@ -480,21 +480,80 @@ const onChangeWithRef = useCallback(
 
 ---
 
-### Commit 2.5b: Extract Multiline Input Editing Handlers
+### Commit 2.5b: Extract Multiline Input Editing Handlers ✅ COMPLETE
 **Files:** `cli/src/components/multiline-input.tsx`  
 **Est. Time:** 3-4 hours  
-**Est. LOC Changed:** ~500-550
+**Actual Time:** ~3 hours  
+**Est. LOC Changed:** ~500-550  
+**Actual LOC Changed:** ~330 insertions, ~240 deletions
 
-| Task | Description |
-|------|-------------|
-| Create `useKeyboardEditing` hook | Backspace, delete, paste |
-| Create keyboard handler registry | Composable handler system |
-| Simplify main component | Delegate all keyboard to hooks |
-| Add comprehensive tests | Cover all key combinations |
+| Task | Description | Status |
+|------|-------------|--------|
+| Create `useTextSelection` hook | Selection management (getSelectionRange, clearSelection, deleteSelection) | ✅ |
+| Create `useTextEditing` hook | Character input, cursor movement, insertTextAtCursor | ✅ |
+| Create `useMouseInput` hook | Mouse click handling, click-to-cursor positioning | ✅ |
+| Extract `TAB_WIDTH` constant | Moved to shared constants file | ✅ |
+| Simplify main component | Delegate editing to hooks | ✅ |
+| Run comprehensive tmux tests | All 6 behavior tests pass | ✅ |
+
+**New Files Created:**
+- `cli/src/hooks/use-text-selection.ts` (~95 lines) - Selection management:
+  - `getSelectionRange` - Get current selection in original text coordinates
+  - `clearSelection` - Clear the current selection
+  - `deleteSelection` - Delete selected text
+  - `handleSelectionDeletion` - Handle selection deletion with onChange callback
+- `cli/src/hooks/use-text-editing.ts` (~140 lines) - Text editing operations:
+  - `insertTextAtCursor` - Insert text at current cursor position
+  - `moveCursor` - Move cursor to new position
+  - `handleCharacterInput` - Handle printable character input
+  - `isPrintableCharacterKey` - Check if key is printable character
+- `cli/src/hooks/use-mouse-input.ts` (~95 lines) - Mouse handling:
+  - `handleMouseDown` - Click-to-cursor positioning with tab width support
+
+**Shared Constant Extraction:**
+- Moved `TAB_WIDTH = 4` to `cli/src/utils/constants.ts` (was duplicated in 2 files)
+
+**Component Size Reduction:**
+- `multiline-input.tsx`: ~560 → ~320 lines (-240 lines, -43%)
+- **Total reduction from original:** ~1,102 → ~320 lines (-71%)
+
+**Test Results:**
+- 1,911 CLI tests pass
+- TypeScript compiles cleanly
+- 6 tmux behavior tests pass (typing, insertion, word deletion, line deletion, emacs bindings, submit)
+
+**Review Findings (from 4 CLI agents):**
+- ✅ Extraction boundaries well-chosen with clear responsibilities
+- ✅ All editing behavior exactly preserved
+- ✅ No dead code or unused exports
+- ⚠️ Warning: TAB_WIDTH duplicated → Fixed by extracting to constants.ts
+- ⚠️ Warning: useMouseInput doesn't use stateRef pattern (acceptable for mouse events)
+- ⚠️ Optional: Remove backwards-compat re-export (tests have own copy)
+- ⚠️ Optional: Type renderer/scrollbox interfaces properly
+
+**Warning Fixes Applied (Amended to Commit):**
+
+After initial commit, 4 CLI agents reviewed and identified warnings. All were fixed and amended to the commit:
+
+| Warning | Problem | Fix Applied |
+|---------|---------|-------------|
+| **Render-time ref update** | `stateRef.current = {...}` runs during render | Documented as intentional for sync state access |
+| **Eager boundary computation** | Word/line boundaries computed for every keypress | Converted to lazy getters (`getWordStart()`, `getLogicalLineEnd()`, etc.) |
+| **shouldHighlight callback churn** | Callback recreated on every keystroke | Memoized with `useMemo` |
+| **TAB_WIDTH duplication** | Defined in multiline-input.tsx and hooks | Removed from component, imports from constants.ts |
+| **useMouseInput missing stateRef** | Didn't use stateRef pattern like other hooks | Updated to use `stateRef` + `onChangeWithRef` |
+| **Type safety ('as any' casts)** | Fragile dependencies on OpenTUI internals | Created `cli/src/types/opentui-internals.ts` with proper interfaces |
+
+**New Type Definitions (`cli/src/types/opentui-internals.ts`):**
+- `TextRenderableWithBuffer` - Text buffer access interface
+- `RendererWithSelection` - Selection management interface  
+- `ScrollBoxWithViewport` - Scroll viewport interface
+- `FocusableNode` - Focus management interface
 
 **Dependencies:** Commit 2.5a  
 **Risk:** Medium  
-**Rollback:** Revert both 2.5a and 2.5b together
+**Rollback:** Revert both 2.5a and 2.5b together  
+**Commit:** `ff968c8c3`
 
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