It also needs to call the downstream function no matter what. These functions should only expand, not contract (or zero out) the indicated region.

It also needs to call the downstream function no matter what. These functions should only expand, not contract (or zero out) the indicated region.

But then the downstream function will colorize that region, won't it? This will result in inconsistent syntax highlighting and flickering once lsp-mode gets semantic tokens from lsp-server. We don't really want to get colors from the major mode here.

But then the downstream function will colorize that region

Yes, but otherwise you are "hogging" jit-lock all for yourself, and other packages which need access to jit-lock updates simply won't work. The way eglot semantic and semel are solving this is to decorate text with semantic-based text-properties as/when available, then, in font-lock (just font-lock-keywords as Stefan is recommending) turn those into 'face. So if there is no semantic info yet available, font-lock wins, but once available, lsp wins.

Honestly, I don't think lsp should need to advise font-lock-fontify-region-function at all; it can just use keywords. See this discussion for more: https://lists.gnu.org/archive/html/emacs-devel/2025-10/msg00024.html.

Note that indent-bars does need to wrap the f-l-f-r-f, because (with scope highlighting), fontification depends on point, which is unusual. So rather than constantly redoing all of font-lock as you scroll point around in the buffer, we arrange to update only the bars (unless the rest of font-lock is actually necessary).

sorry, I just read this today; sounds like I was using the wrong approach but if I can get rid of having to interfere with f-l-f-r-f I'm all for it. @jdtsmith thanks for the pointer(s), I plan to have a look at that proper approach (the one taken by eglot, semantic and semel) around Sunday

@kurnevsky I pushed a commit to my fork at sebastiansturm/lsp-mode that uses font-lock-keywords. Like @jdtsmith said, this seems to be much simpler and work at least as well. But I haven't tested this much and have only read some of the docs on font-lock-keywords so when I find the time next week I should probably do some more poking around before I open a PR with this. But if you can test this and check whether it solves your current issue, that'd be much appreciated

@sebastiansturm just tested your changes, it solves the problem with indent-bars :)

thanks :) I'll open a PR then (not today but I think I should get to it over the weekend)