tablet_pad: fix crash on unmap

If a view is unmapped, its unmap might be delayed by a transaction. In
that case, the wlr_surface may be destroyed while the surface appears
mapped => crash in tablet pad which is based on wlr_surface

src/core/seat/tablet.cpp

@@ -459,6 +459,8 @@ wf::tablet_pad_t::tablet_pad_t(wlr_input_device *pad) :
     on_button.connect(&wlr_tablet_pad_from_input_device(pad)->events.button);
     on_strip.connect(&wlr_tablet_pad_from_input_device(pad)->events.strip);
     on_ring.connect(&wlr_tablet_pad_from_input_device(pad)->events.ring);
+
+    on_focus_destroy.set_callback([&] (auto) { update_focus(nullptr); });
 }
 
 void wf::tablet_pad_t::update_focus()
@@ -487,6 +489,12 @@ void wf::tablet_pad_t::update_focus(wlr_surface *focus_surface)
         wlr_tablet_v2_tablet_pad_notify_enter(pad_v2, attached_to->tablet_v2, focus_surface);
     }
 
+    on_focus_destroy.disconnect();
+    if (focus_surface)
+    {
+        on_focus_destroy.connect(&focus_surface->events.destroy);
+    }
+
     old_focus = focus_surface;
 }
 

src/core/seat/tablet.hpp

@@ -123,6 +123,7 @@ struct tablet_pad_t : public input_device_impl_t
     nonstd::observer_ptr<tablet_t> attached_to;
 
     wf::wl_listener_wrapper on_attach, on_button, on_strip, on_ring;
+    wf::wl_listener_wrapper on_focus_destroy;
 
     wf::signal::connection_t<wf::input_device_added_signal> on_device_added;
     wf::signal::connection_t<wf::input_device_removed_signal> on_device_removed;