flan-check-memory painted every buffer visiting a file the answer named
but decided whether it was already on by looking at the current one.
Asking again from a third buffer therefore stacked a second copy on the
two that were marked, with a second help-echo, and the toggle in the
third buffer took nothing down. Both the toggle and the repaint now go
through the whole buffer list.
The comment beside the faces claimed a dotted underline. The gc face
draws a wave, the same style the error face does; what actually makes
these fainter is that neither inherits error and neither draws its
message into the line. Say that instead, in flan.el and in MANUAL.md.
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