Popups parented to the same toplevel are not stacked properly

[Supreeeme]

In both Niri and Anvil, if you have two popups that are parented to the same toplevel, the second one is stacked under the first one, but it should be stacked above. This works as expected on labwc (wlroots) and KDE.

According to the xdg_popup spec:

A newly created xdg_popup will be stacked on top of all previously created xdg_popup surfaces associated with the same xdg_toplevel.

This is easily reproducible via xwayland-satellite and gtk4-widget-factory. See Supreeeme/xwayland-satellite#153.