With multiple subsurface, only last created (topmost) subsurface and parent get pointer input

This issue occurs in Anvil or cosmic-comp, and probably other compositors.

Window::surface_under calls under_from_surface_tree, which uses from_surface_downward. Then:

The filter is called on the parent surface. It sets found, since the pointer is within the input region of the parent surface.
The post_filter isn't called yet; it's called after iterating over subsurfaces.
It then iterates through subsurfaces in stacking order. Starting with the most recently created subsurface, if place_above/place_below haven't been used to change the stacking order. This may or may not set found, depending on whether or not the pointer is over the subsurface.
At the end of the recursive call, the post_filter is invoked, and returns false. Since found has been set (either to the parent surface, or the first subsurface)
It never gets to the remaining subsurfaces.

So there seem there are two issues here:

from_surface_downward iterates over subsurfaces before calling the post_filter, which doesn't seem to be how under_surface_from_tree expects the function to behave. Is this is correct for from_surface_downward, under_surface_from_tree could be changed to use TraversalAction::SkipChildren when found is set.
But more fundamentally: it doesn't seem to have any way to handle stacking order between subsurfaces and the parent surface
- wl_subsurface::place_above and wl_subsurface::place_above mention that "The reference surface must be one of the sibling surfaces, or the parent surface", so I think it needs to keep track of which subsurfaces are above and below the parent, and iterate over those above the parent before giving pointer focus the parent surface?

Not sure how correct the other functions dealing with the surface tree are. More testing could be helpful; not sure what tests wlcs has for that.

Smithay / smithay