Fix Apple Pencil drawing latency by using Listener instead of GestureDetector

[Copilot]

Drawing with Apple Pencil on iPadOS 26.1 exhibits severe latency—strokes visibly lag behind the stylus. Root cause: GestureDetector waits ~100ms for gesture disambiguation before firing callbacks.

Changes

Replaced GestureDetector with Listener in paint_canvas.dart for immediate pointer event handling:

• Pointer event handlers: onScaleStart/Update/End → onPointerDown/Move/Up/Cancel
• Multi-touch detection: Track _activePointerCount to cancel drawing during pinch-to-zoom
• Tap detection: Manual threshold check (_tapDistanceThreshold = 10.0) to preserve polygon mode tap behavior
• Eraser refactor: _processEraserInput(ScaleUpdateDetails) → _processEraserInputAt(Offset)

// Before: GestureDetector adds disambiguation delay
GestureDetector(
  onScaleStart: _onScaleStart,
  onScaleUpdate: _onScaleUpdate,
  onScaleEnd: _onScaleEnd,
  // ...
)

// After: Listener responds immediately to pointer events
Listener(
  onPointerDown: _onPointerDown,
  onPointerMove: _onPointerMove,
  onPointerUp: _onPointerUp,
  onPointerCancel: _onPointerCancel,
  // ...
)

Multi-touch gestures are detected by counting active pointers; drawing is disabled when _activePointerCount > 1 to allow pinch-to-zoom to pass through.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Bug]: iPadOS 26.1 – Apple Pencil: severe drawing latency in editor</issue_title>
<issue_description>### Package Version

11.15.5

Flutter Version

3.38.6

Platforms

iOS

How to reproduce?

Hi Alex,

we’re seeing an issue on iPadOS 26.1 (tested on iPad Pro 1st–3rd gen and the new iPad 7th gen):

When drawing with the Apple Pencil in the image editor, strokes are rendered noticeably delayed—lines “catch up” after the user has already finished.

This causes a laggy, offset experience and makes precise work difficult.

On Android, the behavior is much better / barely noticeable.

Questions:
Are there known causes/workarounds (e.g., pointer events, requestAnimationFrame, coalesced events, pressure/tilt handling, canvas batching)?
Would a screen recording and device/browser details help you reproduce?

Thanks in advance,
Ali

Logs (optional)

Example code (optional)

Device Model (optional)

iPad Pro iOS version 26.1</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: iPadOS 26.1 – Apple Pencil: severe drawing latency in editor #748

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