bug#57434: 28.1.91; Terminal Emacs Mac OS flickering.

[Dmitrii Kuragin]

I am having difficulty running a debugger.

I tried gdb and signing, but it didn't work. I also tried lldb, but it doesn't stop on a breakpoint for whatever reason.

I compiled with `-O0 -g3`, then

```
lldb
(lldb) file nextstep/Emacs.app/Contents/MacOS/Emacs
Current executable set to '/Users/kuragin/Desktop/emacs/nextstep/Emacs.app/Contents/MacOS/Emacs' (x86_64).
(lldb) breakpoint set -f scroll.c -l 270
Breakpoint 1: where = Emacs`do_scrolling + 485 at scroll.c:271:11, address = 0x0000000100032da5
```

But, it doesn't stop there...

and I run Emacs like this:

```

arch --x86_64 make configure="CFLAGS='-O0 -g3'" -j 20 && nextstep/Emacs.app/Contents/MacOS/Emacs -nw

```

I can confirm that my patch fixes the problem for me, but I am not confident that the issue is in the estimation cost. 

When I have line numbers enabled, I assume, the scrolling logic would always try to insert/delete/write lines. In my case it might be:

 - Writing (Is that writing on top of the current lines?) is cheaper.

 - Screen flickers because of the specific frequency of the terminal (or the way we flush the buffer).

   For example, we insert empty lines and then the screen is updated, only then we add content in there and redisplay again.

Potentially, some redrawing might happen inside of `ins_del_lines`? Instead of redrawing the whole screen, we redraw it in the middle of modifying it?

Those are just my assumptions from reading the code.

I'd appreciate any help in debugging the issue.

On Sat, Aug 27, 2022 at 9:14 AM Eli Zaretskii <eliz@gnu.org> wrote:

> From: Dmitrii Kuragin <kuragin@google.com>
> Date: Sat, 27 Aug 2022 09:01:22 -0700
> Cc: Gerd Möllmann <gerd.moellmann@gmail.com>,
>       57434@debbugs.gnu.org
>
> I think I can give it a try. I just need a bit more time to set up the debugger workflow, since I run GDM only once
> in a few years :)

Thanks.

> I also do not really understand the meaning of "cost" here and what metrics we use to measure that. But, I'd
> assume it is something empirical.

It's supposed to measure the cost of moving the text-terminal cursor
from one point on the screen to another.  And yes, it's heuristics.

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