When you press alttab and an application has got the pointer grabbed, nothing happens instead of switching to the next window. This might be related to display.c line 3054: /* FIXME: * If we have no MetaWindow we do our best * and try to do the grab on the RootWindow. * This will fail if anyone else has any * key grab on the RootWindow. */ if (window) grab_xwindow = window->frame ? window->frame->xwindow : else grab_xwindow = screen->xroot; What happens is that do_change_window() in keybindings.c calls meta_display_begin_grab_op() which fails because the pointer is already grabbed. The attached patch fixes this by just switching to the next window in the tablist when meta_display_begin_grab_op() fails, so this doesn't fix the problem completly (keeping alt pressed still won't work), but it is better then doing nothing at all. A real life example of this problem is xmame, which when requested by the user will grab the mouse, this is actually kinda handy when playing games. xmame will release the grab when it looses focus, this works fine with other wm's but fails with metacity because of the above reasons.
Created attachment 103284 [details] patch
p.s. also submitted upstream: http://bugzilla.gnome.org/show_bug.cgi?id=151500 But that you already know.
Closing here, simpler to just track it at gnome.org.