If two X11 windows were the last two, we'd remove them from the stack
while unmanaging them. That'd hit an assert in
meta_stack_tracker_restack_managed(), resulting in the following crash
when Xwayland exited unexpectedly with two or more X11 windows being the
only windows on the stack:
#1 g_assertion_message() at ../glib/gtestutils.c:3256
#2 g_assertion_message_expr() at ../glib/gtestutils.c:3282
#3 meta_stack_tracker_restack_managed() at ../src/core/stack-tracker.c:1210
#4 on_stack_changed() at ../src/core/stack.c:142
#5 _g_closure_invoke_va() at ../gobject/gclosure.c:895
#6 g_signal_emit_valist() at ../gobject/gsignal.c:3456
#7 g_signal_emit() at ../gobject/gsignal.c:3606
#8 meta_stack_changed() at ../src/core/stack.c:265
#9 meta_stack_remove() at ../src/core/stack.c:324
#10 meta_window_unmanage() at ../src/core/window.c:1542
#11 meta_x11_display_unmanage_windows() at ../src/x11/meta-x11-display.c:111
#12 meta_x11_display_dispose() at ../src/x11/meta-x11-display.c:141
#13 g_object_run_dispose() at ../gobject/gobject.c:1448
#14 meta_display_shutdown_x11() at ../src/core/display.c:831
The added test specifically checks that this scenario is handled
gracefully.
Related: https://bugzilla.redhat.com/show_bug.cgi?id=2143637
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2704>
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