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User callbacks must always return 0 unless they intend to swallow the event to which they are responding. To swallow the event, the callback should return 1.

Only user input, such as mouse click and keypress events, and commit events can be swallowed. If swallowed, no more callbacks are called for that event. If a user input event is swallowed, the user’s mouse click or keypress is ignored. If a commit event is swallowed, it is not placed into the GetUserEvent queue.

Note  The following events can be swallowed: EVENT_COMBO_BOX_INSERT EVENT_COLLAPSE EVENT_COMMIT EVENT_DRAG EVENT_DROP EVENT_END_TASK EVENT_EXPAND EVENT_HSCROLL (Swallowable for tabs only.) EVENT_KEYPRESS EVENT_LEFT_CLICK EVENT_LEFT_DOUBLE_CLICK EVENT_MARK_STATE_CHANGE EVENT_MOUSE_WHEEL_SCROLL EVENT_RIGHT_CLICK EVENT_RIGHT_DOUBLE_CLICK EVENT_SELECTION_CHANGE (Swallowable for trees only.) EVENT_SORT