Confusing feedback when posts are too long

[cellio]

https://meta.codidact.com/posts/287292

Posts are limited to 30,000 characters. It appears that this is a limit on the rendered HTML, not the Markdown source. The post editor can report a "post too long" error while showing a character count that is lower than 30,000 (this is what happened in the linked bug report).

I don't know how expensive it is to check the rendered length during editing. We're showing a preview, so there is HTML; is that in a string or buffer that can be easily checked? If so, we could change the messaging to refer to rendered/output text, including something like "HTML size:" before that character count below the post editor.

I assume we're doing this based on the HTML because the database has to know how large that could be as part of the DDL and HTML is bigger than Markdown, right?

From #909 (duplicate):

Related: if we do the check against the rendered HTML, it would also address https://meta.codidact.com/posts/290859 .

[ArtOfCode-]

I think this is easy enough, but haven't checked. IIRC there's a validation on both the body and body_markdown fields for 30k chars - we'd just need to extend the validation on the body (HTML) field to, say, 35k to allow for the extra generated content.

[trichoplax]

This appears to be a duplicate of #909 (mentions the same Meta post). Not sure which one to close (that one has an extra link to another post, this one has discussion). Would it be easiest to edit this one to include the extra info from the original and then close the original?