inform_compositor_of_window_geometry() calls xdg_surface_set_window_geometry()
unconditionally, but layer-shell surfaces (e.g. those from `kitten panel`) have
no xdg_surface. With hide_window_decorations set to titlebar-only, the panel hits
this geometry call during creation and dereferences the NULL proxy, crashing in
wl_proxy_get_version() with SIGSEGV.
Guard on window->wl.xdg.surface: layer-shell surfaces manage geometry via the
layer surface, so the xdg call is skipped. Normal toplevels are unaffected.
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