I found #11718 which this PR is related to, seems like bram did not get the point of something like this. However it has noticeably reduces flickering for me, so I guess there is a point.

I think this is ready

out_str((char_u *)"\033[?2026$p");

Hm, we typically have those as t_<letter> options. Don't we need this here as well instead of hard coding this?

This PR fixes the popup flickering, but does it as a side effect of the feature, also since it only does it for terminal that supports this feature, I think that having correctness back for popups without transparencies is important. So opaque popups don't require terminals with this feature to avoid rendering issues, while popups with transparencies will require it for avoiding flickering.

Due to this, I think that even with this PR, the with changes from #19534 are still required. Said this, I think it's a great feature to have 👍 .

out_str((char_u *)"\033[?2026$p");

Hm, we typically have those as t_<letter> options. Don't we need this here as well instead of hard coding this?

Well it looks to me that all terminals that support synchronized output use this same escape sequence. However I think it is a good idea to add a t_ option for consistency

The synchronized output does not use termcap to check if it is available, so I don't think we need a t_ option, but maybe for the other escape sequences would be a good idea.

Okay I have made it so the escape codes to begin and end synchronized output are configurable by the user. I left the \033[?2026$p escape code hardcoded, because all terminals that support synchronized output use this, and it makes things simpler (esp. when detecting the response).

Does this the same as in Neovim? If yes, I assume we cannot change the naming? Because if I hear termsync I associate this setting with :terminal. But that is not the case.
On the other hand, this is not true:

:h 'term<c-d>
'term'           'termbidi'       'termwinkey'     'termwinsize'    'termwintype'    'termencoding'   'termguicolors'  'termwinscroll'

Does this the same as in Neovim? If yes, I assume we cannot change the naming? Because if I hear termsync I associate this setting with :terminal. But that is not the case.

On the other hand, this is not true:

:h 'term<c-d>

'term'           'termbidi'       'termwinkey'     'termwinsize'    'termwintype'    'termencoding'   'termguicolors'  'termwinscroll'

I named it termsync because that's what it is named in Neovim. However I suppose we can change the name to something like syncout. What do you think?

let's keep it the same as in Neovim

let's keep it the same as in Neovim

Yes. Since it is essentially a control for the terminal, I think the term prefix is ​​fine.

I think this is ready

thanks