If more than one ScrollView was present previous to this change, the same
assertion (ScrollController attached to multiple ScrollViews.) would be thrown.
Now, on desktop platforms, users need to specify primary: true to
designate which ScrollView is the fallback to receive unhandled keyboard
Shortcuts.

Migration guide

Code before migration:

// These side-by-side ListViews would throw errors from Scrollbars and

// ScrollActions previously due to the PrimaryScrollController.

Scaffold

body:

LayoutBuilder

builder:

context

constraints

return

Row

children:

SizedBox

height:

constraints

maxHeight

width:

constraints

maxWidth

child:

ListView

builder