Done with the split-view mode for the main window. It's one of those things that everyone takes for granted and that, in reality, maps to quite a bit of work if done right.
For example, resizing window by the top edge should resize the top pane and leave the bottom part alone.
Likewise, resizing by the bottom edge should expand/compress the bottom pane, but not the top one.
However, if we are compressing window by the top edge and the top pane is at its minimal height, then we should the bottom pane starting to compress.
All good so far, but, say, we change our mind and start expanding the window back (without releasing the mouse button).
We should see the bottom pane brought back to its original size first, followed by the expansion of the top part... even though we are, technically, dragging by the top edge.
This sort of UX detail is something that few people will notice if it's done right, because it feels natural and aligns well with expected behavior. But cut some corners - and you bet