GlCollapsibleListbox header size varies based on popup height
The height of the dropdown header changes between these two examples, even though the code for unassign and select all is the same.
The button has a height of 24px, and so that's the most natural content size of the header. Since the popup has a maximum height, when there are lots of options, the header (flex) shrinks below that, to the minimum content size (the text height, I suppose). So you can make the right hand example look like the left hand one by setting flex-shrink: 0 on the header.
By setting flex-basis: 0 on the left one, we're forcing the button's height to be ignored in the basis calculation - or something like that!
Desired behaviour
Both options look like the right example, smaller header.
References
Edited by Dan MH