Two holes left by the last change. A frame is opened with `window.location.href`
rather than a link, so the studio was still handed a fresh page and the scan with
it — every route out of the catalogue now goes through `go()`, which pushes the
address instead of reloading while a scan is in flight. And a row counted what was
filed away rather than what had been read, so it sat at zero for the whole of a
first scan: the catalogue's frames only reach IndexedDB in one batch at the end.
`ScanProgress.counts` carries the frames the scan has reached, per row, off the
scan's own bookkeeping, and the column reads it while the scan runs — the row
under the reader's eye moves as the roll is read, and the toolbar's line stays the
whole picture. A re-read counts from the start, which is what it is doing.
The regression check grows the two: a frame opened mid-scan from the catalogue
(of a roll it already holds) has to stay in the page, and the row has to count.
3dtours
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