The same three regions, but the one you are reading is given a width instead of taking one — and the width nobody wants goes to a region with nothing in it.

A region given its width

`zone--narrow` with a size step: the region is that wide and stays that wide, whatever room the window has. It is how a text is kept readable — a column that grows with the screen stops being one.

What the slack is for

The last region holds nothing, and keeps its ground all the same: empty is not absent, and a pane waiting reads as a pane rather than as a hole. It is there to take the width the sized ones refuse.

A split hands the leftover to whatever asks for it. With nothing asking, the sized regions would be stretched or pushed to the edges; with this, they keep their width and the page keeps the rest.