layout: add test that Stack doesn't allocate

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

layout/stack_test.go

@@ -29,3 +29,21 @@ func TestStack(t *testing.T) {
 		t.Errorf("Stack ignored Expanded size, got %v expected %v", got, exp)
 	}
 }
+
+func TestStackAllocs(t *testing.T) {
+	var ops op.Ops
+	allocs := testing.AllocsPerRun(1, func() {
+		ops.Reset()
+		gtx := Context{
+			Ops: &ops,
+		}
+		Stack{}.Layout(gtx,
+			Stacked(func(gtx Context) Dimensions {
+				return Dimensions{Size: image.Point{X: 50, Y: 50}}
+			}),
+		)
+	})
+	if allocs != 0 {
+		t.Errorf("expected no allocs, got %f", allocs)
+	}
+}