I'm using Geany 0.21 on Linux.
This feature would be very useful. E.g. I often need to work in the embedded VTE terminal at (almost) full screen. I currently have to drag the frame border all the way up, then again all the way down. If one could just focus the terminal window, then double click its top frame border in order to maximize it and double-click it again to return to its previous dimension, that would be much faster and easier. Today most people use touchpads with tapping support instead of traditional mouses, so double clicks are even silent :)