In a corner case, the toggling of the popup menu entered a state
where the menu could not be opened anymore by the user. Specifically
the following input sequence triggered this problem.
1. The user opens the menu
2. The user clicks on the menu and holds the button
3. While holding the button, the user moves the pointer to the
outside of the popup (e.g., to the '+' button)
4. The user releases the button.
In this situation, the popup is closed but the hover information for the
popup contains still the original clicked-on item. Hence, all subsequent
clicks on the '+' appear as both a click on the '+' (opening the popup)
and a click on the "hovered" popup entry (closing the popup).
The patch explicitely clears the popup's hover information when closing
the popup.
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