widget/material: fix list scrollbar display

This commit fixes a visual misalignment in scrollbars resulting from subtle differences
in the semantics of layout.Stack and layout.Background. layout.Stack will position expanded
children according to their minimum constraint regardless of their returned size, whereas
layout.Background uses their returned size. This means that layout.Expanded widgets returning
zero dimensions are positioned correctly, but they break when converted to use layout.Background.

This commit fixes the problem by returning correct dimensions from the scrollbar track.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/material/list.go

@@ -165,9 +165,6 @@ func (s ScrollbarStyle) layout(gtx layout.Context, axis layout.Axis, viewportSta
 	if axis == layout.Horizontal {
 		inset.Top, inset.Bottom, inset.Left, inset.Right = inset.Left, inset.Right, inset.Top, inset.Bottom
 	}
-	// Capture the outer constraints because layout.Stack will reset
-	// the minimum to zero.
-	outerConstraints := gtx.Constraints
 
 	return layout.Background{}.Layout(gtx,
 		func(gtx layout.Context) layout.Dimensions {
@@ -186,10 +183,9 @@ func (s ScrollbarStyle) layout(gtx layout.Context, axis layout.Axis, viewportSta
 			s.Scrollbar.AddTrack(gtx.Ops)
 
 			paint.FillShape(gtx.Ops, s.Track.Color, clip.Rect(area).Op())
-			return layout.Dimensions{}
+			return layout.Dimensions{Size: gtx.Constraints.Min}
 		},
 		func(gtx layout.Context) layout.Dimensions {
-			gtx.Constraints = outerConstraints
 			return inset.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 				// Use axis-independent constraints.
 				gtx.Constraints.Min = axis.Convert(gtx.Constraints.Min)