The width of the area within which a horizontal swipe will open the drawer.
By default, the value used is 20.0 added to the padding edge of
MediaQuery.of(context).padding that corresponds to the surrounding
TextDirection. This ensures that the drag area for notched devices is
not obscured. For example, if
TextDirection.of(context) is set to
TextDirection.ltr, 20.0 will be added to
final double drawerEdgeDragWidth