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Ah, GTK+. I wanted to receive mouse-up events, so I connected a callback to the "button-release-event" event, and nothing happened. I added GDK_EVENT_BUTTON_RELEASE_MASK to my widget's event mask, and still nothing happened. Then I added GDK_EVENT_BUTTON_PRESS_MASK, and even though I didn't have a callback for it, my button-release events started firing.