I've added another fix, this removes most of the height problems for me, with the fix I just have maybe 20px of scrolling sometimes when I drag the handles while before it jumped between no scrollbars inside the panes and too small panes. The problem was that the height handling code was triggered before the width handling code and thus the right pane was below the other panes during the height calculations which caused strange effects. Now the height handling code is called again after adjusting the width of the right pane.
In Firefox I have the feeling that both the old and the new version of the resizing is a bit slow which might be explained by what's happening between the things you can see in the browser - the right pane jumps below the other panes and back up again and in each state we have our height handling code run which does different things in the two states. I think for future releases it would be good to look again at this code and maybe find some way to resize the right pane earlier or to prevent it from jumping down in the first place. Maybe we could also run the height handling code less often/only after a delay of some ms.