One click too many when a plugin is shown and another one is clicked on
When a certain panel plugin is clicked on and opened (Calendar or Directory Menu for example) and user wants to open a different one, two clicks are required for that. The first one for closing the current menu and the second one for opening the new one. A more practical behavior would be if only one click was required (on the newly selected plugin icon) to both close the current one and open the next one at once.
The Whisker Menu plugin actually already solved that, requiring only a single click to open another panel plugin even when its menu is opened. The best solution would probably be to transfer its solution to a more general part of the code so it applies to all plugins (and possibly between panels as well).