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

[Dmitrii Kuragin via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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`tty->TS_ins_line` (al) is supprted.
`tty->TS_ins_multi_lines` (AL) is supprted.
`tty->TS_del_line` (dl) is supprted.
`tty->TS_del_multi_lines` (DL) is supprted.

to verify that I used `tput <cap_name>`.

I think, that is sufficient for `tty->line_ins_del_ok` to be true, but fo
completeness:

`tty->TS_fwd_scroll` (sf) is supprted.
`tty->TS_rev_scroll` (sr) is supprted.


`tty->TS_set_window` (wi) is NOT supprted.
`tty->TS_set_scroll_region` (cs) is supprted.
`tty->TS_set_scroll_region_1` (cS) is NOT supprted.

BUt I do not know how to verify `tty->Wcm->cm_abs`.


```
tty->scroll_region_ok
    = (tty->Wcm->cm_abs
       && (tty->TS_set_window || tty->TS_set_scroll_region ||
tty->TS_set_scroll_region_1));
```


```
tty->line_ins_del_ok
    = (((tty->TS_ins_line || tty->TS_ins_multi_lines)
        && (tty->TS_del_line || tty->TS_del_multi_lines))
       || (tty->scroll_region_ok
           && tty->TS_fwd_scroll && tty->TS_rev_scroll));
```

BTW, here's a video with what I have with "-Q", it still flickers:
https://drive.google.com/file/d/1Yq2QFWHR6CHkoM4buEokV6p3a1uOI8ao/view?usp=sharing

On Tue, Aug 30, 2022 at 4:09 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Gerd Möllmann <gerd.moellmann@gmail.com>
> > Cc: Eli Zaretskii <eliz@gnu.org>,  57434@debbugs.gnu.org
> > Date: Tue, 30 Aug 2022 08:09:33 +0200
> >
> > Dmitrii Kuragin <kuragin@google.com> writes:
> >
> > > So far:
> > > ```
> > > :~/Desktop% tput al; echo $?
> > > 0
> > > :~/Desktop% tput AL; echo $?
> > > 1%dL0
> > > :~/Desktop% tput dl; echo $?
> > > 0
> > > :~/Desktop% tput DL; echo $?
> > > 1%dM0
> > > :~/Desktop% tput sf; echo $?
> > >
> > > 0
> > > 0~/Desktop% tput sr; echo $?
> > > :~/Desktop% tput wi; echo $?
> > > tput: unknown terminfo capability 'wi'
> > > 4
> > > :~/Desktop% tput cs; echo $?
> > > %p1%d;%p2%dr0
> > > :~/Desktop% tput cS; echo $?
> > > tput: unknown terminfo capability 'cS'
> > > 4
> > > ```
> >
> > Same here.
>
> Thanks.
>
> But I'm quite confused by all of this, because you don't show all the
> relevant capabilities, AFAICT.
>
> We have in term.c:
>
>   tty->scroll_region_ok
>     = (tty->Wcm->cm_abs
>        && (tty->TS_set_window || tty->TS_set_scroll_region ||
> tty->TS_set_scroll_region_1));
>
>   tty->line_ins_del_ok
>     = (((tty->TS_ins_line || tty->TS_ins_multi_lines)
>         && (tty->TS_del_line || tty->TS_del_multi_lines))
>        || (tty->scroll_region_ok
>            && tty->TS_fwd_scroll && tty->TS_rev_scroll));
>
> Please try all of the relevant capabilities and tell me which ones are
> supported and which aren't.  (Please also mention both the capability
> string and its term.c flag name, so that I shouldn't need to jump
> back-and-forth in the source looking up each one to understand what it
> means.)
>
> Then we have in dispnew.c:
>
>   /* If we cannot insert/delete lines, it's no use trying it.  */
>   if (!FRAME_LINE_INS_DEL_OK (f))
>     inhibit_id_p = 1;
>   [...]
>   /* Try doing i/d line, if not yet inhibited.  */
>   if (!inhibit_id_p && i < desired_matrix->nrows)
>     force_p |= scrolling (f);
>
> Which means that 'scrolling', and thus 'scrolling_1' (where the
> problem happens) will not be called if the line_ins_del_ok flag is not
> set.
>
> Furthermore, we have in scrolling_1:
>
>   if (FRAME_SCROLL_REGION_OK (frame))
>     {
>       calculate_direct_scrolling (frame, matrix, window_size,
>                                   unchanged_at_bottom,
>                                   draw_cost, old_draw_cost,
>                                   old_hash, new_hash, free_at_end);
>       do_direct_scrolling (frame, frame->current_matrix,
>                            matrix, window_size, unchanged_at_top);
>     }
>   else
>     {
>       calculate_scrolling (frame, matrix, window_size, unchanged_at_bottom,
>                            draw_cost, old_hash, new_hash,
>                            free_at_end);
>       do_scrolling (frame,
>                     frame->current_matrix, matrix, window_size,
>                     unchanged_at_top);
>     }
>
> which means do_direct_scrolling (which causes the problem) will not be
> called if the terminal's scroll_region_ok flag is not set.
>
> So given all of this, can you tell whether Emacs does TRT here?  That
> is:
>
>   . are all the capabilities that are supposed to be available for
>     these two flags are indeed available?
>   . do we need to check any additional capabilities, which are in fact
>     used in the problematic scenario, but not tested as part of
>     setting these two flags?
>
> Assuming that Emacs does TRT, i.e. sets the flags correctly and uses
> only the capabilities that are conditions for the flags set, then the
> next important question is: why doesn't Gerd see the flickering on a
> very similar system and the same terminal emulator?  Is it possible
> that some other local software configuration on Dmitrii's machine
> causes this, directly or indirectly?  E.g., Dmitrii, do you have some
> display-related software/driver that has some "optimization" features
> turned on?  If so, can you turn them off and try again?
>
> Thanks.
>


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