macOS: sometimes content lingers after resize when alpha

[erf]

I'm not able to recreate this issue consistently, but here are steps to reproduce:

1. i use a kitty.config with "background_opacity 0.8"
2. i do a "ls" to display some text
3. i rapidly resize the window using the bottom right corner
   Sometimes when i do this the font enlarges and changes ratio, and sometimes when this happens, the stretched / enlarged text is left as part of the background ( screen is not completly cleared it seem ).

I'm on macOS, kitty version 0.13.0.

Screenshot of artifact after resize ( notice the white enlarged text in the background ):

[kovidgoyal]

I cannot reproduce this on my macbook. Indeed during a resize, the kitty window is explicitly rendered with an empty background color, so I dont see how this can happen. This was done deliberately, see 21d586c

[erf]

I usually have to give this a few (~10) tries before the issue shows up. Resizing from small to big.

[kovidgoyal]

I tried resizing it continuously big smal smal big for a few minutes ata time and could not reproduce.

[erf]

Ok. I also notice this, on a rare occasion, happens when i press CTRL-l to clear the screen, and the text lingeres in the background as grey text. Maybe it could be a threading/timing issue.

[kovidgoyal]

dont see how, kitty is single threaded, only input from the child process is read in a different thread. All actual rendering happens in one thread.