Sidebar scrollbar is glitched

A quick way to reproduce is to do the following:

1. Open libhandy example
2. Open GtkInspector
3. Switch to CSS tab, type in:

stacksidebar row {
  padding: 100px 0;
}

This will make sidebar rows very tall, so that a scrollbar appears

4. Hover over the scrollbar

Expected result can be seen in any other app. :)