bug#24372: 25.1.50; After losing focus, cursor is hidden when moving point

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sun, 11 Sep 2016 09:15:49 +0000
> Cc: 24372@debbugs.gnu.org
> 
>  > OK, I guess one issue is that setting blink-cursor-delay doesn't restart blink-cursor-idle-timer.
>  (Similarly,
>  > changing blink-cursor-interval doesn't restart blink-cursor-timer.) While obviously we can't fix that
>  when using
>  > setq, I'd suggest adding custom setters to the variables nevertheless.
> 
> I've attached a patch for this. It shouldn't be controversial because it only reduces the possibility for surprises,
> but doesn't change any behavior.

Using the maximum of blink-cursor-delay and blink-cursor-interval
effectively removes the user's ability of controlling the delay before
the cursor starts blinking when Emacs becomes idle, doesn't it?  If
so, I don't think this could qualify as not changing any behavior.
Plus, if the user sets the interval to a very small value, we have the
same problem again.

How about a variant of this below?  It uses a fixed limitation from
below on the delay, but only for the first blink.  (The value 0.2 was
found by experimentation, not sure if we need to add yet another
defcustom for that.)

> It does introduce the adverse side effect that now the first blink takes one second (the sum of
> cursor-blink-delay and cursor-blink-interval).

Right.

> I've attached another patch with the change I have in mind.

This has a disadvantage of creating a new timer object each time,
which I think we'd like to avoid: too much consing.  (Also, don't you
need to set the timer variable to nil when the timer is disabled?)

I'd prefer something along the lines of the first idea, the patch
below or some variant of it.  It is simpler.

diff --git a/lisp/frame.el b/lisp/frame.el
index cfd40bf..4540172 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2114,7 +2114,7 @@ blink-cursor-check
 	     (not blink-cursor-idle-timer))
     (remove-hook 'post-command-hook 'blink-cursor-check)
     (setq blink-cursor-idle-timer
-          (run-with-idle-timer blink-cursor-delay
+          (run-with-idle-timer (max 0.2 blink-cursor-delay)
                                blink-cursor-delay
                                'blink-cursor-start))))
 
@@ -2148,7 +2148,7 @@ blink-cursor-mode
     (add-hook 'focus-in-hook #'blink-cursor-check)
     (add-hook 'focus-out-hook #'blink-cursor-suspend)
     (setq blink-cursor-idle-timer
-          (run-with-idle-timer blink-cursor-delay
+          (run-with-idle-timer (max 0.2 blink-cursor-delay)
                                blink-cursor-delay
                                #'blink-cursor-start))))