floating-overlay-reserves-its-own-strip

A control floated over a scrolling column is a sibling that takes part in no measurement, so the column has to reserve the strip it covers explicitly — as a leading spacer, or as a header row whose twin spacers keep a centred title centred over the hole. Covers what the strip constant may and may not count, why every scrolling branch needs its own, and why floating is what keeps the control reachable once the page…