A fixed-width box is now sized like one, and a code-hosting site gets its sidebar back

A box with a fixed width contributes exactly that width when a shrink-to-wrap parent measures how much room it needs - a rule so basic it is easy to miss that it was wrong. The engine had been measuring such a box at the width of its text instead, so a fixed-width panel inside an auto-width wrapper became invisible to that wrapper's sizing. Found by looking at a code-hosting site: a repository page's whole right-hand column - About, topics, stars and forks, Releases, Sponsor - was wrapping off the bottom of the page and out of view, because the sidebar's fixed width never reached the flexbox that was supposed to reserve room for it, so the file list took the entire row. With the fix the sidebar sits beside the file list exactly as in the reference browser. The rule is careful about the cases where a width is not a fixed number - a table whose columns can push past it, a percentage that needs a container to resolve against, an image with its own intrinsic size - and leaves those alone. The whole engine gate holds at 6375 of 6375 and no test page regressed.