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

• Go on a long English Wikipedia page, like this one
• Ensure that &safemode=1 is added in the url to disable all gadgets and user scripts
• Edit it in source mode with the newer wikitext bar
• Scroll all the way at the bottom
• Try to type some text

What happens?:

The text appears a couple of lines below the cursor

What should have happened instead?:

Typed text should appear directly before the cursor

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

See attached video for how to reproduce.