The same number read straight from the DOM. gutter is React state, and a
fit can run in the same task as the ResizeObserver callback that recomputes
it — the reveal of this page from display: none is exactly that moment.
That setGutter has not committed, so a fit reading the prop sees the
hidden column's 0 and frames the whole canvas, leaving the rightmost nodes
under the panels for the session. Measure, don't remember.
Canvas pixels covered by the panel column, so a fit frames the visible strip.