OpenGL terminals: At times visualizer flickers

[poetaman]

Describe the bug
OpenGL based GPU accelerated terminals like Alacritty, WezTerm, etc update screen differently compared to older terminals. Occasionally while playing music the Visualizer screen seems to flicker. The problem could be similar to Vim's way of redrawing the screen or just that there is some glitch in the pipeline of frames to display. vim/vim#8002. Or check this, the developer fixed their redrawing mechanism: valebes/rsClock#4 (comment):

The problem might not occur at times for entire song, and at times it happens more often... That is unlike rsclock, or vim, though more close to rsclock's behavior in the video at valebes/rsClock#4 (comment)... I think adding a little bit of transparency to the background might also help in observing it.

Modern GPU based terminals draw the entire frame on redraws. The way to avoid screen from stuttering is to use a different approach to redrawing screen.
The solution is mentioned in neovim/neovim#14225 (comment), and vim/vim#8002 (comment). The solution is to redraw with
NOT_VALID, which would redraw everything but not clear the screen.

To Reproduce
Steps to reproduce the behavior:
Open the default visualizer in an openGL terminal like Alacritty or WezTerm (IMO the problem is more often visible in WezTerm, but can't say for sure).

Expected behavior
At least not flicker the screen.

Screenshots/Logs
If applicable, add screenshots or logs to help explain your problem.

Desktop (please complete the following information):

• macOS 11.5.2 Darwin ARM
• Alacritty/WezTerm, zsh
• ncmpcpp --version: ncmpcpp 0.9.2

[poetaman]

One of the reason screens flicker is that they are cleared instead of drawn again... not sure if that is what could be going on?