The NSEventTypeLeftMouseUp event is never sent to the NSView when there is an active NSDraggingSession. NSEventTypeLeftMouseDragged events are sent and processed just fine, but the mouse up event is never sent.

1. Build and run the app
2. Click and drag from anywhere in the window to anywhere in the window
3. View the Console in Xcode
4. Notice that "mouseup" is never printed to the console

After running above repro steps, the console should show:

1. "mousedown" once
2. "mousedragged" multiple times during the drag
3. "mouseup" when the drag is complete