I have noticed that when I have foliage tab up for painting mesh instances the contents of it cannot be scrolled up when your contents go below the available window space.
It is especially annoying when youre on 768 pixels screen height, so you have to maximise it as much as possible - and even so you can only choose to use one of the the mesh selection or the object properties segments as they each only have a very small portion available to you, since the top part of the foliage tab cannot be scrolled up. The mesh selection area or the mesh options area can both be resized and scrolled, but not the foliage toolset above it.