main: Unset the right prevFocus actor after the focus stack got shifted

When a modal that's not on top of the modalActorFocusStack gets popped,
we shift the focus stack as described in popModal() to ensure the chain
remains correct. That however destroys the association of a modal actor
and its prevFocus actor on the focus stack, because the prevFocus actors
are now moved to different entries of the stack.

Now when a prevFocus actor gets destroyed, we don't handle that case
correctly and search for the modal actor that was associated with the
prevFocus actor before the stack was shifted, which means we end up
unsetting the wrong prevFocus actor.

So fix that and search the stack for the prevFocus actor which is being
destroyed instead to unset the correct entry.

Thanks to Florian Müllner for figuring out the actual issue and
proposing this fix.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2446

js/ui/main.js

@@ -539,7 +539,9 @@ function pushModal(actor, params) {
     let prevFocusDestroyId;
     if (prevFocus != null) {
         prevFocusDestroyId = prevFocus.connect('destroy', () => {
-            let index = _findModal(actor);
+            const index = modalActorFocusStack.findIndex(
+                record => record.prevFocus === prevFocus);
+
             if (index >= 0)
                 modalActorFocusStack[index].prevFocus = null;
         });