A band starts from the space between things, so that a drag across words
still selects words to copy. On a page that is wall-to-wall text there is
no such space, and the gesture simply did nothing — which is how it failed
on a real document at 276% zoom, where every candidate starting point had
a line of text under it.
Alt-drag starts a band anywhere, text or not. The select hint says so.
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