The event should return a 'gboolean', indicating whether the event
needs propagating any further. We were returning void, which meant
that the default handling might be accidentally suppressed.
On Wayland, this had the particularly nasty effect that window redraws
would stop completely if you maximised the terminal window.
(By trial and error I found that this stopped happening if I removed
GDK_HINT_RESIZE_INC from the geometry hints, from which I guess that
the default window-state-event handler is doing something important
relating to that hint, or would have been if we hadn't accidentally
suppressed it. But the bug is clearly here.)
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