Fills a border with quads. Selects the newly created faces (sometimes you want to do an inset afterwards) and verts (sometimes you might want to relax them).
Run the .mcr file once. It creates a polyOp Border Fill item in the polyOp category under Customize UI, which you can then add to a quadmenu or assign a shortcut to it (that's what I'm showing in the preview gif).
To use it, select a border vertex that defines the border corner and press the button/keyboard shortcut - this will fill a regular border.
Or select two corners and press the shortcut, when filling irregular border. The border edge count has to be even.