From BlenderWiki

I will be posting some proposals here for the splitting and resizing of regions.

Pop-up Button

When the user moves their cursor within the threshold of the region's borders a pop-up with one or more icons appears. The most common option given the current context (ie. Resize when the border is within the client region and Split when the border is on the edge of the client area) will be directly under the cursor, this will provide a zero-cost solution for the user expecting to resize the region and splitting is a simple case of moving the mouse over the correct icon, clicking and dragging.