Scrolling dashboard down shows grey backgroud

[amisevsk]

Describe the bug

When on a dashboard page that is longer than the current window, scrolling down shows a grey background where the sidebar is:

Che version

• latest
• nightly
• other: please specify

Steps to reproduce

Open a workspace config, navigate to the plugins tab, and scroll the page down.

Expected behavior

Either the background should continue with the same color, or the sidebar should scroll with the window.

Screenshots

Environment

Reproducible on Chrome and Firefox.

Additional context

Reported in #13942 (comment)