widget,widget/material: use constraints for setting up hit area

Before this change, the widget.Button.Layout method assumed the caller had set
up the pointer hit area before. Further, the very common rectangular hit
areas needed both an AreaOp and a widget.Button.Layout call.

Make widget.Button less subtle and more useful by setting up a
pointer hit area given by the incoming minimum constraints.

Drop a pointer.AreaOp made redundant by the change.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/button.go

@@ -3,10 +3,12 @@
 package widget
 
 import (
+	"image"
 	"time"
 
 	"gioui.org/f32"
 	"gioui.org/gesture"
+	"gioui.org/io/pointer"
 	"gioui.org/layout"
 	"gioui.org/op"
 )
@@ -44,7 +46,11 @@ func (b *Button) History() []Click {
 func (b *Button) Layout(gtx *layout.Context) {
 	// Flush clicks from before the previous frame.
 	b.processEvents(gtx)
+	var st op.StackOp
+	st.Push(gtx.Ops)
+	pointer.Rect(image.Rectangle{Max: gtx.Constraints.Min()}).Add(gtx.Ops)
 	b.click.Add(gtx.Ops)
+	st.Pop()
 	for len(b.history) > 0 {
 		c := b.history[0]
 		if gtx.Now().Sub(c.Time) < 1*time.Second {

widget/material/button.go

@@ -116,7 +116,6 @@ func (b ButtonLayoutStyle) Layout(gtx *layout.Context, button *widget.Button, w
 			})
 		}),
 		layout.Expanded(func() {
-			pointer.Rect(image.Rectangle{Max: gtx.Constraints.Min()}).Add(gtx.Ops)
 			button.Layout(gtx)
 		}),
 	)