(Tested in Firefox 54 and Chrome 60)
Steps to reproduce:
- Open Draft:Sandbox with the visual editor.
- (optional) Select something
- Click either the hyperlink or the cite button -> a popup balloon appears
- Click either the hyperlink or the cite button (doesn't need to be same one)
- The top border of the popup balloon is visible.
- The editor is stuck and unusable, new changes are ignored by the save button (may be related to T159224)