Default behavior of C-l causes flickering on TTY frames

Default behavior of C-l causes flickering on TTY frames

[Daniel Martín]

When Emacs runs on a TTY console, the common operation of recentering
the window with C-l causes some unpleasant screen flickering most of the
time.  After some investigation, I see that this happens because the
default value of the variable 'recenter-redisplay' is 'tty', which means
that C-l clears and redisplays the screen.

Is there a reason Emacs still does that by default on TTY frames?  Is
the possibility of the screen becoming garbled so common these days (on
TTY frames) that we pay the price of a redisplay on each C-l?

Re: Default behavior of C-l causes flickering on TTY frames

[Eli Zaretskii]

> From: Daniel Martín <mardani29@yahoo.es>
> Date: Sun, 12 Dec 2021 14:39:37 +0100
> 
> 
> When Emacs runs on a TTY console, the common operation of recentering
> the window with C-l causes some unpleasant screen flickering most of the
> time.  After some investigation, I see that this happens because the
> default value of the variable 'recenter-redisplay' is 'tty', which means
> that C-l clears and redisplays the screen.
> 
> Is there a reason Emacs still does that by default on TTY frames?

Yes, it's to redraw a potentially garbled TTY display.

> Is the possibility of the screen becoming garbled so common these
> days (on TTY frames) that we pay the price of a redisplay on each
> C-l?

Yes.

But why don't you customize that variable if you dislike the redraws
and OTOH don't have the problem with garbled displays?

Re: Default behavior of C-l causes flickering on TTY frames

[Daniel Martín]

Eli Zaretskii <eliz@gnu.org> writes:

>
> But why don't you customize that variable if you dislike the redraws
> and OTOH don't have the problem with garbled displays?

Yes, I know.  I suppose there's a lot of variability in the
implementation quality of terminals, so this default cannot be changed.

I don't use terminal Emacs a lot, but I always thought the
flickering was a bug or a poor implementation somewhere, because I
didn't experience the same flickering effect in other terminal-based
text editors.

Re: Default behavior of C-l causes flickering on TTY frames

[Eli Zaretskii]

> From: Daniel Martín <mardani29@yahoo.es>
> Cc: emacs-devel@gnu.org
> Date: Sun, 12 Dec 2021 15:51:41 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >
> > But why don't you customize that variable if you dislike the redraws
> > and OTOH don't have the problem with garbled displays?
> 
> Yes, I know.  I suppose there's a lot of variability in the
> implementation quality of terminals, so this default cannot be changed.

It's not just the terminals.  It's enough to C-z out of Emacs and run
something that messes up the terminal to need this feature.  It is
also consistent with other programs that have similar features: Bash,
GDB, etc.: they clear the display when you type C-l.  So having this
in Emacs requires less Emacs-specific muscle memory.

> I don't use terminal Emacs a lot, but I always thought the
> flickering was a bug or a poor implementation somewhere, because I
> didn't experience the same flickering effect in other terminal-based
> text editors.

It flickers _only_ when you manually type C-l, not when the function
is invoked from Lisp.  So it's you yourself who causes the flickering,
so to say ;-)