A click on the next frame of the roll emptied the stage before the bytes
were read — `clearPreview` for the picture, and `loadFile` dropping the
catalogue's frame id for the strip — and both the roll and the frame's own
info line lived inside the branch that drew the picture. The whole strip
went with it and was drawn again on the way back: every tile torn down and
read a second time, three loads and three fresh images, for a click that
changed one frame.
The info line and the roll stand outside that branch, so the roll survives
the click, and a caller opening a frame of the catalogue's own roll says so
and keeps the id `loadFile` otherwise clears. The info line now sits above
the roll instead of under it.
probe-strip-click.mjs, desktop and phone: 0 tiles torn down, 0 loads,
the frame clicked is the one lit, the info line above the roll.
3dtours
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