Idea Summary
Region splitter like the one in designer as a widget will be so useful for screen with many regions which are to be shown in a single view-port. User can resize the region in runtime as per need on which he/she is focused right now.

Use Case
A dashboard having multiple regions which are to be presented simultaneously to make decision, not in tab/collapsible.

Preferred Solution (Optional)
Just provide what you have implemented in the designer. It should have horizontal and vertical splitter with full expansion and drag -shift.