layout: compute Position.Offset correctly for ScrollToEnd Lists

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

layout/list.go

@@ -266,11 +266,11 @@ func (l *List) layout(ops *op.Ops, macro op.MacroOp) Dimensions {
 	}
 	l.Position.Count = len(children)
 	l.Position.OffsetLast = mainMax - size
-	pos := -l.Position.Offset
 	// ScrollToEnd lists are end aligned.
 	if space := l.Position.OffsetLast; l.ScrollToEnd && space > 0 {
-		pos += space
+		l.Position.Offset -= space
 	}
+	pos := -l.Position.Offset
 	layout := func(child scrollChild) {
 		sz := l.Axis.Convert(child.size)
 		var cross int

layout/list_test.go

@@ -25,6 +25,24 @@ func TestEmptyList(t *testing.T) {
 	}
 }
 
+func TestListScrollToEnd(t *testing.T) {
+	l := List{
+		ScrollToEnd: true,
+	}
+	gtx := Context{
+		Ops:         new(op.Ops),
+		Constraints: Exact(image.Pt(20, 10)),
+	}
+	l.Layout(gtx, 1, func(gtx Context, idx int) Dimensions {
+		return Dimensions{
+			Size: image.Pt(10, 10),
+		}
+	})
+	if want, got := -10, l.Position.Offset; want != got {
+		t.Errorf("got offset %d, want %d", got, want)
+	}
+}
+
 func TestListPosition(t *testing.T) {
 	_s := func(e ...event.Event) []event.Event { return e }
 	r := new(router.Router)