GtkStack does not respect its child CSS padding/margin during transition
In Gtk4, GtkStack forgets to take into account the CSS padding/margin value of its child during the transition. That means that the child “bumps” a bit at the start of the (sliding, in my case) transition. Things work correctly with the “margin-start/end/top/bottom” properties of the widget, and it worked correctly in Gtk3.
(I solve that easily in my applications by adding a level of GtkBox as the child of the GtkStack, but anyway.)