Fix two-finger pinch-to-zoom in reader modes (#256)

[aaronbamblett]

Closes #256.

The bug

Reported in #256: two-finger pinch is "almost unusable" on Android — you have to "find the right angle" before it triggers. In practice it works once in ten attempts, never reliably.

Root cause

Gesture-arena race. The reader wraps content in DirectionalSwipeGestureHandler, whose HorizontalDragGestureRecognizer / VerticalDragGestureRecognizer accept the gesture as soon as either finger drifts past drag-slop (~18px). Meanwhile InteractiveViewer's scale recognizer is waiting for an actual scale delta to claim. The drag recognizer almost always wins, eating the pinch. Fast pinches occasionally race through because the scale delta accumulates faster.

Verified on a Pixel 9 Pro with an in-app diagnostic counter overlay (since stripped): scale callbacks fired zero times during slow pinches, while the outer drag recognizer's end fired every time.

The fix

Two pieces:

1. Replace InteractiveViewer with zoom_view. Suggested by @DattatreyaReddy in the issue thread itself. It's a Flutter package designed for the "zoom inside a scrollable" case — handles gesture-arena coordination between scale and scroll via a shared controller.

2. Small HorizontalDragGestureRecognizer / VerticalDragGestureRecognizer subclasses in DirectionalSwipeGestureHandler that reject multi-touch. When a second finger lands, the recognizer rejects its in-flight gesture via resolve(GestureDisposition.rejected) so the scale recognizer can claim. Single-finger swipes (the swipe-at-chapter-boundary feature) are unchanged. Tap and long-press handlers stay on a nested standard GestureDetector since they don't compete with multi-touch.

Configured zoom_view's forceHoldOnPointerDown: true flag (recommended by the package for use inside ScrollablePositionedList) and doubleTapDrag: true for the standard double-tap-to-zoom affordance.

Dependency situation (please read)

ScrollablePositionedList doesn't expose a standard ScrollController — zoom_view needs one to coordinate scroll position with zoom. There's an open upstream PR at google/flutter.widgets#535 (by yakagami, the same author as zoom_view) that adds the needed ScrollPosition getter on ScrollOffsetController — 2-line change. It's been open since June 2024.

Pending that merge, this PR pins scrollable_positioned_list to yakagami's fork via git URL in pubspec.yaml. There's a comment in pubspec linking to PR #535 noting the situation; once Google's PR lands, the line can be reverted to a pub.dev version.

If you'd rather not introduce a git dep, the alternative is to vendor the 2-line change into a local copy of the package. Happy to take that path instead — just say the word.

Tests

Three widget tests under test/src/widgets/zoom/ cover the recognizer behavior:

• Single-finger drag still triggers the end callback (didn't break the swipe-at-boundary feature).
• Two-finger drag does NOT trigger the end callback (the actual fix).
• After a multi-touch gesture, a fresh single-finger drag still works (the recognizer isn't left in a broken state).

One important note: the widget-test gesture arena is more permissive than the real-device arena. The test fixture deliberately includes a no-op ScaleGestureRecognizer so the arena has competition — without it, the lone drag recognizer eager-accepts and masks the bug. There's a documentation-only test calling this limitation out explicitly.