widget/material: propagate ButtonLayout minimum constraints to content

The previous change fixed a regression where minimum constraints larger than 0
would not affect the button. This change moves the minimum constraints one
level lower so the content widget will see them as well. The wrapping
layout.Center ensures that any misbehaving widgets still end up centered.

Add a test to lock in the new behaviour and the previous fix.

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

widget/material/button.go

@@ -109,9 +109,9 @@ func (b ButtonLayout) Layout(gtx *layout.Context, button *widget.Button, w layou
 			}
 		}),
 		layout.Stacked(func() {
-			gtx.Constraints.Width.Min = hmin
-			gtx.Constraints.Height.Min = vmin
 			layout.Center.Layout(gtx, func() {
+				gtx.Constraints.Width.Min = hmin
+				gtx.Constraints.Height.Min = vmin
 				b.Inset.Layout(gtx, func() {
 					paint.ColorOp{Color: b.Color}.Add(gtx.Ops)
 					w()

widget/material/material_test.go

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package material
+
+import (
+	"image"
+	"testing"
+
+	"gioui.org/layout"
+	"gioui.org/widget"
+)
+
+func TestButtonLayout(t *testing.T) {
+	var gtx layout.Context
+	gtx.Reset(nil, image.Point{X: 100, Y: 100})
+
+	ButtonLayout{}.Layout(&gtx, new(widget.Button), func() {
+		if got, exp := gtx.Constraints.Width.Min, 100; got != exp {
+			t.Errorf("minimum width is %d, expected %d", got, exp)
+		}
+		if got, exp := gtx.Constraints.Height.Min, 100; got != exp {
+			t.Errorf("minimum width is %d, expected %d", got, exp)
+		}
+	})
+}