FAB jumps when scaffolds have different geometry

[MatrixDev]

I have two screens with each having a different ScaffoldPrelayoutGeometry. This causes FAB to jump sporadically when switching screens.

This is caused by the static/shared/global ExpandableFab.location, which stores scaffold geometry in its field:

@immutable
class ExpandableFab extends StatefulWidget {
  /// The location of the ExpandableFab on the screen.
  static final FloatingActionButtonLocation location = _ExpandableFabLocation();

  // ...
}

class _ExpandableFabLocation extends StandardFabLocation {
  final ValueNotifier<ScaffoldPrelayoutGeometry?> scaffoldGeometry =
      ValueNotifier(null);

  // ...
}

Because scaffoldGeometry is a field in a static instance and updated by different scaffolds, this causes multiple FABs fighting for last-one-wins scaffold geometry, which causes constant jumping during animation.

[uyenbuidev]

You can fix by create a custom MyExpandableFabLocation class and use it instead:

class MyExpandableFabLocation extends StandardFabLocation {
  final ValueNotifier<ScaffoldPrelayoutGeometry?> scaffoldGeometry =
      ValueNotifier(null);
  @override
  double getOffsetX(
    ScaffoldPrelayoutGeometry scaffoldGeometry,
    double adjustment,
  ) {
    Future.microtask(() {
      this.scaffoldGeometry.value = scaffoldGeometry;
    });
    return 0;
  }

  @override
  double getOffsetY(
    ScaffoldPrelayoutGeometry scaffoldGeometry,
    double adjustment,
  ) {
    return 10;
  }
}