On changes that insert/remove lines, we redraw the entire gutter, which
ensures the foldability markers are up to date. But other kinds of
changes could change the foldability of lines… if a line becomes
commented or its indentation changes, the foldability of that line and
the lines above and below could be affected. For now, the gutter
understands this fact and updates the foldability for the region
extending from one line before to one line after each change. Someday
I'd like the view logic to not bake in knowledge of the folding rules,
but this works for now.
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