Cannot access preview content when Design menu is open [25 January 2021]

The the Design menu is open on any 7.0 or 7.1 site, you can no longer select anything in the preview window. This occurs when any Design panel is open.

When the Design menu is closed, everything returns to normal.

Although this issue was fixed on 26 January 2021 for most Design menu options, the issue persists when the Custom CSS panel is open.

Fixed 12 February 2021 but reappeared two weeks later.

Resolved on 09 July 2021.