fix(tui): delay initial clear until we know the bg color and termguicolors value

[bfredl]

Note: I haven't been able to reproduce the issue myself (pangoterm rendering go brrr), but It might help with the case where termguicolors is changed during startup.

[IngoMeyer441]

I can confirm that your patch is working. 👍 I tested it in Kitty with termguicolors set.

[gpanders]

Even without this patch, but with #26579 now in master, I can no longer reproduce the original issue from #26372, even when artificially introducing a delay using nvim --cmd 'sleep 1'.

[bfredl]

yes, #26579 has the same effect as this w.r.t the visual change. This PR (as a replacement) would only be required if it is necessary to check the detected value of termguicolors early even if it doesn't take effect (I can't think of a good reason, but it is what it would do)

[bfredl]

after discussing a bit on matrix, we concluded always setting 'termguicolors' as a OptionSet event is simpler (rather than having it sometimes being pre-set and sometimes being delayed to after startup, depending on the exact timing). So anyone who cares about the detected value should use an OptionSet event anyways.

[bfredl]

so, this might still be relevant for cases where the user set termguicolors manually. (my problem is that even my craptop is fast enough to never see this kind of flicker, after all rendering a terminal is peanuts compared to surfing the modern web, so I wouldn't really know )

[IngoMeyer441]

Without this patch, I can still recognize flicker in tmux in Kitty 0.31.0. There is a black frame for some milliseconds, followed by the correct background color. With this patch, I never see the black background.