bug#34022: 26.1; Region highlight has lower priority than flymake overlays

[João Távora]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: João Távora <joaotavora@gmail.com>
>> Cc: Johann Klähn <johann@jklaehn.de>, 34022@debbugs.gnu.org
>> Date: Fri, 11 Jan 2019 09:33:33 +0000
>> 
>> In Flymake multiple overlays (error, warning, note) can overlap and
>> priorities are used so that an error is never hidden by a note.  I based
>> thosed priorities in the 100's range (so note = 101, warning=102,
>> error=103, sth like that). 100 was a number chosen quite arbitrarily
>> which worked fine until Johann noticed the bug (I do think it's a bug).
>
> Would it still DTRT if you use priorities around 50?

No.  You can see the problem for yourself (Johann had already pasted a
slightly harder to follow recipe):

  Emacs -Q
  M-x flymake-mode RET
  M-: (set-face-background 'flymake-note "yellow green") RET
  C-x C-f lisp/progmodes/flymake.el RET
  M-g M-g 1057 RET

Case A: Start marking a region from the beginning of the line.
Case B: Start marking a region from the middle of the line

With stock flymake.el, in both cases the region is invisible in the
parts where it overlaps the green diagnostic.  Actually you could sort
of say that in case A it's doing the right thing.  But in case B it
clearly isn't.

Also if I make Flymake use priorities around 50 as you suggest (actually
41 in this case).

But if I make Flymake use priorities of the form (nil . 41), it is
possible to see both the region and the diagnostic in both A and B.  I
think it's the way this should behave.

>> Perhaps it isn't.  Can you point me to other packages that leave
>> persistent region-resisting overlays in the buffer?
> Wht do you mean by "region-resisting"?
> What is the meaning of "playing nice" in this context?

I hope I've clarified above!

João