When I copy something and paste into the summary field (over a selection), and the content is truncated due to limitations in the summary length, I'm unable to undo the action and get the old content back.

Steps to reproduce

1. Open a random page in edit mode: https://en.wikipedia.org/wiki/Special:Random?action=edit&summary=DEFAULT&uselang=en
2. Copy a string of text from somewhere (e.g. the copyright notice above the editbox - I used a triple click to select it, followed by CTRL+C)
3. Click after the last character in the edit summary field (or select the whole default summary)
4. Paste the copied string as many times as needed so it is truncated (e.g. for the copyright notice, 2 times is enough)
5. Attempt to undo the paste to restore the DEFAULT summary (I used CTRL+Z for this)

Actual result
It is not undone

Expected result
The original DEFAULT summary should be restored.