perf: fix redundant docs rendering

[eero-lehtinen]

After 05229dd adding the use of stylize_markdown, I consistently experience half second stutters when docs are open and switching between items. Nvim-cmp has zero issues with this.

Basically I copied what nvim-cmp does, which removed all performance issues for me. The buffer is cleared more efficiently. nvim_buf_set_lines is not used redundantly only to be instantly overridden by stylize_markdown. Setting the "modified" option is moved to the end of the function.

[scottmckendry]

I've A-B tested doc performance with both this PR and main and there definitely an improvement here, however slight.

I did notice the doc output looks different. For example, this is main:

And with your changes:

The latter looks better, IMO. Any idea why this is?

Edit: just realised this is the exact problem that this PR is addressing 🤦 LGTM!

[eero-lehtinen]

I'm not sure why main looks different to you. For me this PR looks the same.

The lag seems to be much more pronounced with rust-analyzer for some reason. With lua_ls it didn't really make a difference. I also tested on another machine with the same config and there was less lag than on my main machine, so maybe it's more complicated.

The lag is also much worse on nvim nightly.

main with nvim nightly (After the first 6 secs I start tapping quickly and it stutters badly):

Screencast_20241015_123926.webm

this PR with nvim nightly (I'm tapping just as quick and there is no stutter):

Screencast_20241015_124018.webm

It doesn't show that well on video because KDE Spectacle seems to add its own lag but it feels way better.

[Saghen]

Thanks for finding this regression! I wasn't happy with the performance even if we cached it and noticed that it was using regex highlighting instead of treesitter. I took inspiration from the vim.treesitter.highlighter code to highlight the documentation window ourselves. It has a high initial cost of 50ms for compiling the markdown query, but afterwards, it updates in 1-8ms. Lmk if performance looks good on your end

[eero-lehtinen]

Performance feels great now. Also looks better with treesitter. Thanks!