[BUG] Tap relative offsets are incorrect when the widget is within a scaling widget

[bramp]

What is the bug?

When the map is inside a scaling widget, such as Transform.scale(... child: FlutterMap(...)) the onTap gestures are not scaled correctly, leading to both the onTap(TapPosition tapPosition, LatLng point) being slightly the wrong place, even though the map looks correct.

This seems to be due to PositionedTapDetector2._getTapPositions translating the global coordinates to local coordinates, but not correctly scaling them.

How can we reproduce it?

testWidgets('Test tap is detected where expected with scale and offset.',
      (tester) async {
    const screenSize = Size(400, 400);
    await tester.binding.setSurfaceSize(screenSize);

    Offset? lastTap;
    // The Transform.scale fills the screen, but the PositionedTapDetector2
    // occupies the center, with height of 200 (0.5 * 400) and width 400.
    final widget = Transform.scale(
      scaleY: 0.5,
      child: PositionedTapDetector2(
        onTap: (position) => lastTap = position.relative,
        child: SizedBox(
          width: screenSize.width,
          height: screenSize.height,
          child: Container(
            color: Colors.red,
          ),
        ),
      ),
    );

    await tester.pumpWidget(widget);

    // On the screen the PositionedTapDetector2 is actually 400x200, but the
    // widget thinks its 400x400.
    expect(screenSize, tester.getSize(find.byType(SizedBox)));

    Future<void> tap(Offset pos, Offset expected) async {
      lastTap = null;
      await tester.tapAt(pos);
      expect(lastTap, expected);
    }

    // Tap top left of PositionedTapDetector2 which should be 0,0.
    await tap(const Offset(0, 100), Offset.zero);

    // Tap bottom right of PositionedTapDetector2
    await tap(const Offset(400 - 1, 300 - 0.5),
        Offset(screenSize.width - 1, screenSize.height - 1));
  });

Do you have a potential solution?

Yes, #1676

Platforms

Tested on Chrome (Web) and Android.

Severity

Obtrusive: Prevents normal functioning but causes no errors in the console