Steps to Reproduce:

1. Open Android Chrome 73.0.3683.90 or Samsung Browser 9.2.00.70.
2. With these browser, go to any module page in MediaWiki Wiki. If you're using mobile mode, please switch it into desktop mode.
3. Switch into Edit Source mode.
4. With code editor, make new line in any place.

Actual Results: You can find the last charactor of previous line was erased when you create a new line.

If you try the test in test2.wikipedia.org, the result will be same. But in Ace editor kitchen-sink page, the bug won't be occured.

Expected Results: No characters are removed.

I'm sorry for writing with poor English.