Synchronise height of panes

[xtempore]

Just a minor thing, but it'd be really useful if when resizing the panes, the left and right both changed together. Especially when trying to synchronise things, it's helpful to have things lined up, and I regularly find myself adjusting the height of the left browser pane and then trying to adjust the right side to match.
I think most people would want them to remain aligned, so that should be the default action.
There might be odd cases where having them different heights would be useful, but perhaps that could be catered for by holding down the Shift key.
I.e. When moving the horizontal divider, move both equally -- unless Shift is pressed.