The issue is that each button on the control panel now spans the full width of the browser window. So Emergency Off-line, Master Password, Password-protect Administrator, etc. all display on one line each at full width. This makes the Control Panel screen very difficult to read.
When I was looking at previous tickets, I noticed a similar issue under ticket 29174
The referenced ticket is for Akeeba Backup, but it appears to be the same issue. I too see the same problem in the Akeeba Backup Control Panel (I currently have v 6.0.1 Core running). The ticket, 29174, is closed due to age but did not seem to have a resolution other than suggestions to clear the cache. I have tried clearing the Joomla cache and expired cache as well as the browser cache. I have tried running in Incognito mode as well. None of these resolve the display problem. I have now seen this issue on multiple sites I support.