bug#20285: 25.0.50; blink-cursor-mode sometimes stops blinking

[Eli Zaretskii]

> Date: Sat, 11 Apr 2015 10:34:23 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 20285@debbugs.gnu.org
> 
> > Of course, it's still possible that during the 10 seconds where the
> > cursor was invisible on the screen there hasn't been a redisplay of the
> > selected window, but how can I know?
> 
> If your Emacs is configured with --enable-checking='yes,glyphs', then
> you can invoke "M-x trace-redisplay RET", and see every entry to
> redisplay_internal announced on stderr.

After building Emacs 24.5 and observing the same issue there, I've
decided to try to set blink-cursor-interval to something that is not
an integer divider of 1 sec.  So I set it to 0.53 sec, and lo and
behold, the problem disappeared: the cursor blinks and blinks, and
never stops blinking, even when there's JIT Stealth working in the
background, as well as other stuff, as always in my sessions.

So I think at least what I observed is somehow related to the specific
value of 0.5 sec.  Perhaps some other timer of other periodic activity
(auto-save?) that runs at an integral multiple of 0.5 sec gets in the
way somehow.  E.g., it could toggle the cursor twice before the next
redisplay cycle.  Or something.