Bugfix: Unmap windows before reparenting them to the root window (fixes Mathematica) (Thanks psychon)

Some apps such as Mathematica send a synthetic UnmapNotify event without
properly unmapping their window first. This change makes sure that
happens and fixes an annoying bug with Mathematica where some unmanaged
windows would stay around, but you couldn’t do anything with them.

Thanks to psychon (current awesome maintainer) for helping with the
diagnosis!

fixes #787

src/tree.c

@@ -235,7 +235,12 @@ bool tree_close(Con *con, kill_window_t kill_window, bool dont_kill_parent, bool
             return false;
         } else {
             xcb_void_cookie_t cookie;
-            /* un-parent the window */
+            /* Ignore any further events by clearing the event mask,
+             * unmap the window,
+             * then reparent it to the root window. */
+            xcb_change_window_attributes(conn, con->window->id,
+                    XCB_CW_EVENT_MASK, (uint32_t[]){ XCB_NONE });
+            xcb_unmap_window(conn, con->window->id);
             cookie = xcb_reparent_window(conn, con->window->id, root, 0, 0);
 
             /* Ignore X11 errors for the ReparentWindow request.