Titlebar sliding animations are broken

Sliding animations in titlebars are broken, as during the animation:

the middle of the sliding widgets is visible below the window's drawn rounded corners
drawn corners are visible in the middle of the titlebar

Here is the widget hierarchy to reproduce it:

GtkWindow titlebar
- GtkStack with the slide left-right animation
  - GtkHeaderBar
  - GtkHeaderBar

Then just switch between the two header bars in the stack. I did that in GTK+ 3 and I didn't test it yet in GTK+ 4.

The bug was initialy found with libhandy's HdyLeaflet widget: libhandy#21.