• 2D Follow up: Add dual scrollbar constructors and methods #122348
  • will need to be completed first

Part of the 2D scrolling proposal: flutter.dev/go/2D-Foundation

#122349 adds a factory constructor that creates two scrollbars for scrolling in two dimensions. This change is proposed as a lead up to TwoDimensionalScrollable etc.

The dual factory constructor works almost perfectly when plugged into TwoDimensionalScrollable, except for the small overlap in the joining corner.

While investigating this, I found multiple native behaviors. Most common:

Image	Description
	The scrollbars operate on a first-come-first-serve rule for the corner space. Here the horizontal bar got there first, the vertical is not allowed to overlap. Observed on MacOS as default behavior.
	Empty space. No scrollbar enters the corner space.

Both of these should be supported.

I have investigated this a fair bit, and I think the best way for this to work (instead of creating two instances of the scrollbar widget) is to have the .dual constructor instead create two instances of ScrollPainter internal to the single scrollbar widget.

This is because in order to properly avoid overlapping each other, the painters need to know about the state of each other since the scrollbar size and visibility can change based on hovering, dragging, whether or not scrolling is happening, etc.