I found that the Sidebar overlaps the top menu if an 'You have temporary access to administrative functions' message is displayed on top. It also affects the Navigation Menu Editor plugin. However, this bug appears even if you disable this plugin.

Reproduction:

1. Sign in admin panel and return to the wiki (without removing the token).
2. Locate the page where there is a vertical scroll bar.
3. Scroll to the bottom and then to the up using the mouse wheel.

Please find the screenshot in attachment.