Subject: bug#36564: 27.0.50; Wrong number of errors in compilation mode-line

From: Stefan Monnier <[email-masked]>
Date: Mon, 15 Jul 2019 08:44:21 -0400

> IIUC, after adding compilation--ensure-parse there is no
> performance degradation in case when the compilation buffer
> is displayed during compilation, because compilation--ensure-parse
> is called on the same-sized chunks as when the buffer is fontified
> by font-lock.

Only when the window displays the bottom of the buffer.

> I noticed this when tried to debug the problem of fontifying
> diff hunks, but I failed to find a solution.  The problem is this:
> sometimes diff-mode doesn't refine some hunks during font-lock
> when the first part of the hunk emitted by diff-process-filter
> (from the diff command comparing files) is fontified partly,
> then after emitting the remaining part of the hunk it remains
> unfontified.

Yes, it's a bit tricky to handle this right while at the same time
trying to avoid refontifying the same hunk N times (where N is
proportional to the hunk size).

        Stefan