Steps to replicate the issue (include links if applicable):

• Go to this section of my sandbox.
• Edit the source of the section.
• Place the cursor in any position other than the first on the final line (beginning with <!--This-->). Step can be accomplished using either the mouse or the arrow keys.

What happens?:
The cursor's position becomes disjoint from where text is inserted into the article. For example, in the string Foo|bar (| representing the apparent cursor position), pressing l results in Foob|lar.

Inserting an additional character at the end of the previous line results in desired behaviour.

What should have happened instead?:
Fool|bar

Other information (browser name/version, screenshots, etc.):
I'm running Firefox 127.0-mint-001 on Linux Mint.

I think that this bug might be caused by incorrect whitespace wrapping, as there seems to be an undelete-able leading space on the offending line, which vanishes if you try to insert a character before it.

Could this issue have the same underlying cause as T360351 and T355625?