I've done some revisions, might do a few more (like a few more obsolete TODO comments and such) but this is pretty much mergable from my point of view. We have the 0.5 cycle to fix bugs/edge cases (which will be reported by master testers after merge :) . This is is pretty much "do nothing unless invoked", so it should not affect stability in general.

Docs can and will be improved after merge (but of course point out already if there is something obviously wrong/misleading).

I also plan to heavily refactor the internal implementation based on #9404 (a more efficient kbtree that don't need two separate layers for columns/lines) at some point, but this is not critical. There is no catastrophe if this happens after 0.5 (or never).

@zgpio updated to your suggestions (and a bit more).

Some typos in docs.

Thanks, but I have a doc update pending anyway. There are many typos, but we want to avoid lots of typo comments in this PR.

@ashfinal Thanks, but note that method-specific documentation in api.txt is generated, so some of those will be overwritten anyway. Method documentation should be reviewed in src/nvim/api/*.c.

I added comments for the Adjust* structs, but did no changes now. Will try to reduce them later.

Also fixed the segfault on buffer wipe.

Merged. Thanks @timeyyy for this great contribution.

As mentioned before, this just the start of "extmark" feature. Apart from doc/implementation improvements, an follow-up will be to integrate bufhl/virttext into it, which I hope to look into soon.

Thanks for getting this slotted in.

I have a doc update pending anyway.

#11376