window-x11: Accept any focusable window as fallback focus

As per commit f71151a5 we were ignoring WM_TAKE_FOCUS-only windows as focus
targets, however this might end-up in an infinite loop if there are multiple
non-input windows stacked.

So, accept any focusable window as fallback focus target even if it's a
take-focus one (that might not reply to the request).

Added a stacking test to verify this.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/660
https://gitlab.gnome.org/GNOME/mutter/merge_requests/669

(cherry picked from commit c327b2df95)

src/Makefile-tests.am

@@ -19,6 +19,7 @@ dist_stacking_DATA =				\
 	tests/stacking/closed-transient-no-input-no-take-focus-parents.metatest	\
 	tests/stacking/closed-transient-no-input-parent.metatest	\
 	tests/stacking/closed-transient-no-input-parent-delayed-focus-default-cancelled.metatest	\
+	tests/stacking/closed-transient-no-input-parents.metatest	\
 	tests/stacking/minimized.metatest   	\
 	tests/stacking/mixed-windows.metatest   \
 	tests/stacking/set-parent.metatest	\

src/tests/stacking/closed-transient-no-input-parents.metatest

@@ -0,0 +1,46 @@
+new_client 0 x11
+create 0/1
+show 0/1
+
+new_client 1 x11
+create 1/1
+show 1/1
+
+create 1/2 csd
+set_parent 1/2 1
+accept_focus 1/2 false
+show 1/2
+
+create 1/3 csd
+set_parent 1/3 2
+accept_focus 1/3 false
+show 1/3
+
+create 1/4 csd
+set_parent 1/4 3
+accept_focus 1/4 false
+show 1/4
+
+create 1/5 csd
+set_parent 1/5 3
+show 1/5
+
+wait
+assert_focused 1/5
+assert_stacking 0/1 1/1 1/2 1/3 1/4 1/5
+
+destroy 1/5
+dispatch
+
+assert_focused none
+assert_stacking 0/1 1/1 1/2 1/3 1/4
+
+sleep 250
+assert_focused none
+assert_stacking 0/1 1/1 1/2 1/3 1/4
+
+destroy 1/3
+wait
+
+assert_focused none
+assert_stacking 0/1 1/1 1/2 1/4

src/x11/window-x11.c

@@ -852,7 +852,7 @@ meta_window_x11_focus (MetaWindow *window,
                       if (!focus_window)
                         break;
 
-                      if (focus_window->input)
+                      if (focus_window->input || focus_window->take_focus)
                         break;
 
                       if (focus_window->shaded && focus_window->frame)