Normally, one can make a window fill all available vertical space (without changing its width) by moving the cursor to the top/bottom of the window such that it becomes a double-pointed arrow, and double clicking.

With a Groupy'd set of windows, this doesn't work:

• Double clicking at the top of the window will maximise the window (i.e. it'll fill the entire screen, both horizontally and vertically).
• Double clicking at the bottom of the window will usually switch to a different tab, and then do the expected behaviour of filling all vertical space.