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

Also part of this:

• Update dual scrollbar constructor to support corner space #122345
• Update dual scrollbar constructor to support different animation responses #122351

The proposal outlines adding a .dual constructor to the scrollbar classes in order to support 2D scrolling

• RawScrollbar
• Scrollbar
• CupertinoScrollbar

It also proposes adding a method to scroll behaviors for calling on this in the 2D case. Relying on ScrollBehavior again will also enable other options in 2D scrolling, like if the developer does not want one or both of the scrollbars.

• ScrollBehavior
• MaterialScrollBehavior
• CupertinoScrollBehavior