Screenshot of the error

Steps to reproduce:

1.Click on the "Edit" button for a page
2.Type something with a link on the main page
3.Add a template for example:Bronze Age
4.Add another line with a reference
5.Add reflist to the article
6.Save the page
7.Reopen the editor
8.Press Delete (Deletes the first line)
9.Press Delete again (Nothing happens)
10.Press Undo

Observed Result:

1.Deletes all the content of the article.
2.Only the blue empty highlighted box remains at the right
3.Save Page button becomes disabled
4.Make further edit but still the "Save Page" button remains disabled for that session

See the screenshot attached.

Version: unspecified
Severity: normal

Attached: