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The code here assumed a `leave` event always occurs between two `enter` events. On Sway (and presumably other compositors) this happens even if the client has destroyed the `wl_surface`, but the client gets a null `surface` here. (Which presumably on on the wire is the id of the destroyed surface.) This seems like a bad thing to rely on, and is easy to avoid. But if this is correct to assume, the Wayland protocol should be explicit about this. Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1527>