I've gotten the GestureDetectors to be the correct size and position for all of the different text selection handles that we have.

However, despite the GestureDetectors being the right size and in the right place, there are parts of them that are sometimes still not interactive. I think that's because of the CompositedTransformFollower that we use to reset the coordinate space.

From the docs, "Hit testing on descendants of this widget will only work if the target position is within the box that this widget's parent considers to be hitable." I can't quite get a perfect visual on exactly where the widget's boundary is and if it lines up perfectly with the non-interactive regions I'm seeing, though.

Update: Hit detection works in the places we need it now as of dcf30cf.

The last problem that I'm fighting with is that the GestureDetector swallows tap events from the text field behind it. This manifests as double tapping to select text being broken (the first tap shows the handle, which blocks the second tap).

@HansMuller I've got a working solution by using Listener instead of GestureDetector, like we talked about. I was able to reuse the dragging logic from GestureDetector, but I reimplemented tap myself (poorly) because reusing any code caused the event to be swallowed. I'll talk to you on Monday about it, and hopefully I can bypass having to write my own tap if I understand a bit more about the event arena.

@HansMuller This is ready for review. The main things this PR does to achieve its goal of making the handles nicely interactive are:

1. Refactor how handles are drawn so that they can be easily padded to a nice 48x48 interactive size. TextSelectionControls renders the handles in an encapsulated way, and the parent TextSelectionHandleOverlayState handles padding them and putting them in the right place wrapped in a GestureDetector.
2. Changed how the selection overlay uses CompositedTransformFollower and the gesture arena to get interactions to work on both handles and text field at the same time. This was the big time sink as there were many gestures that I needed to get right while still allowing 48x48 interaction with and without overlap of the underlying text field. See my big list of sneaky gestures above.

Also, do you have any idea what is going on with the tests?