Web Viewer displaying strange blue square

This is a bit of a bizarre one which ate 90 minutes of my morning - so I thought I'd share it in case anyone else encounters it.

One of our FileMaker use cases has an embedded web viewer displaying a Google Sheet (it's a runtime Kiosk app but the same issue applies to FMPro). This worked well and the users could update the spreadsheet (a rota).

However this morning I noticed a strange bluey/grey overlay (see attachment). After quite a bit of digging it seems Adobe Flash is still installed in IE 11 and this was causing some form of issue.

I disabled Flash in Group Policy (gpedit.msc) and the problem went away. There are other ways of disabling it which should also work.

This was on Windows 10. I have now tested it on two machines so it should be fairly easily reproducible.

I assume it's somehow caused by the 2nd October update to the Flash control.

Anyway hope this helps if others have the same issue.