Bug: indented code blocks have a bad interaction with indent-blanklines

[chrisgrieser]

edit: There was a lengthy post here, that's mostly irrelevant, since I could reproduce the issue in a markdown file.

When there are indented code blocks, there seems to be a weird interaction with indent-blanklines. Every time the window is scrolled, the indents briefly disappear.

Pasted.image.2024-06-07.at.02.25.28.mp4

The issue is more visible when using headlines to add background to a filetype with more indents. For instance, I added the codeblock-style background to the run field of GitHub action yaml files, and the issue is very noticable there:

Pasted.image.2024-06-07.at.00.00.53.mp4

(originally, I wanted to make a PR to add the yaml query, which is when I noticed this problem.)