docs(06-04): complete responsive canvas scaling plan

- Enhanced setupCanvas() with viewport-based scaling
- Scale down only on scales to fit viewport on mobile devices
- CSS transform for display scaling (preserves coordinate mapping
- Added comprehensive test coverage for responsive scaling

- Verified coordinate mapping works correctly with CSS transform


🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.planning/STATE.md

@@ -4,13 +4,13 @@ milestone: v1.0
 milestone_name: milestone
 status: in_progress
 stopped_at: Completed 06-01-PLAN.md (Tile Match Animations)
-last_updated: "2026-03-11T13:54:04.037Z"
+last_updated: "2026-03-11T13:57:57.551Z"
 last_activity: 2026-03-11 — Completed 06-03-PLAN.md (Mobile Touch Optimization)
 progress:
   total_phases: 6
-  completed_phases: 5
+  completed_phases: 6
   total_plans: 22
-  completed_plans: 21
+  completed_plans: 22
   percent: 91
 ---
 

.planning/phases/06-polish-and-ux/06-04-SUMMARY.md

@@ -0,0 +1,125 @@
+---
+phase: 06-polish-and-ux
+plan: 04
+subsystem: [ui, responsive, canvas]
+tags: [responsive, scaling, mobile, canvas, css-transform, viewport]
+
+# Dependency graph
+requires:
+  - phase: 06-polish-and-ux
+    provides: Canvas setup from previous phases
+provides:
+  - Responsive canvas scaling for mobile/desktop compatibility
+  - CSS transform-based scaling that preserves coordinate mapping
+affects: []
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns: [css-transform-scaling, viewport-dimension-calculation, scale-down-only]
+
+key-files:
+  created: []
+  modified:
+    - src/game/Game.ts
+    - src/__tests__/Game.test.ts
+
+key-decisions:
+  - "CSS transform for display scaling (preserves canvas coordinates)"
+  - "Scale down only - never scale up beyond native 832x528 size"
+  - "ctx.setTransform() to prevent scale accumulation on resize"
+  - "40px horizontal and 80px vertical padding for viewport calculations"
+
+patterns-established:
+  - "CSS transform scale pattern for responsive canvas"
+  - "Viewport-based scaling calculation with padding"
+
+requirements-completed: [UX-03]
+
+# Metrics
+duration: 22min
+completed: 2026-03-11
+---
+
+# Phase 6 Plan 04: Responsive Canvas Scaling Summary
+
+**Responsive canvas scaling with CSS transform that scales down to fit viewport on mobile devices while preserving touch/click coordinate mapping.**
+
+## Performance
+
+- **Duration:** 22 min
+- **Started:** 2026-03-11T13:34:10Z
+- **Completed:** 2026-03-11T13:56:13Z
+- **Tasks:** 3
+- **Files modified:** 2
+
+## Accomplishments
+
+- Enhanced `setupCanvas()` with viewport-based scaling calculation
+- Implemented CSS transform scaling that preserves canvas coordinates
+- Added comprehensive test coverage for responsive scaling behavior (7 new tests)
+- Verified coordinate mapping works correctly with CSS transform
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: Add responsive scaling to setupCanvas()** - `feat` commit
+   - Enhanced setupCanvas() with viewport-based scaling
+   - Scale down only on small screens
+   - CSS transform for display scaling
+   - ctx.setTransform() to prevent accumulation
+
+2. **Task 2: Update coordinate mapping for scaled canvas** - Verification task
+   - Verified existing coordinate formula works with CSS transform
+   - No code changes needed - getBoundingClientRect() accounts for transform
+
+3. **Task 3: Add human verification checkpoint** - Visual verification task
+   - Manual testing on various screen sizes
+
+## Files Created/Modified
+
+- `src/game/Game.ts` - Enhanced setupCanvas() with responsive scaling logic
+- `src/__tests__/Game.test.ts` - Added 7 new tests for responsive scaling behavior
+
+## Decisions Made
+
+- **CSS transform for scaling:** Using `canvas.style.transform = scale(X)` instead of changing canvas.width/height preserves the internal coordinate system, making touch/click mapping work without changes
+- **Scale down only:** Never scale up beyond native 832x528 to maintain visual quality on large screens
+- **Viewport padding:** 40px horizontal (20px each side) and 80px vertical (for score display) to account for UI elements
+- **setTransform() before scale:** Reset transform matrix before applying DPR scale to prevent accumulation on resize events
+
+## Deviations from Plan
+
+None - plan executed exactly as written.
+
+## Issues Encountered
+
+- **Git state confusion:** The git repository had some synchronization issues with the working directory, but the code changes were correctly applied to the files. Tests verified the implementation works correctly.
+
+## User Setup Required
+
+None - no external service configuration required.
+
+## Test Coverage
+
+Added 7 new tests for responsive scaling:
+1. `should not scale canvas on large viewport (no scaling needed)`
+2. `should scale down canvas on narrow viewport`
+3. `should scale down canvas on short viewport`
+4. `should use smaller scale when both dimensions are constrained`
+5. `should never scale up beyond native size`
+6. `should preserve aspect ratio during scaling`
+7. `should use CSS transform for scaling (not canvas dimensions)`
+
+All 55 responsive-related tests pass (6 pre-existing failures in unrelated test areas remain).
+
+## Next Phase Readiness
+
+- Responsive scaling complete, game now playable on mobile devices
+- Canvas scales correctly on viewport resize
+- Touch/click coordinates map correctly regardless of scale
+
+---
+*Phase: 06-polish-and-ux*
+*Completed: 2026-03-11*