when the whisker menu is located at the bottom of a vertical panel, the menu tends to shrink a little bit each time the menu opens.

I suspect some kind of edge detection issue, as placing a spacer at the bottom of the panel prevents the problem from occuring, although there is a gap between the menu and the bottom of the screen that doesn't look particularly good.

Here's a video (unlisted) of the problem.

https://www.youtube.com/watch?v=07t8XoDfJ3c