[philja]

Another day - and a small advance. I left Sigil unattended on an active W10 box until the screensaver kicked in - no problem. I then left it unattended as a non-active virtual desktop with another virtual desktop also active. I waited at least an hour and no problem.

I then left the W10 box to KVM over and use the linux box for half an hour. When I went back to the W10 box, Sigil had lost its menubar.

I tried the first item on Kevin's list : minimised the Sigil mainwindow and then restored it and back came the menubar.

Once the menubar was back, I checked the Advanced settings just to confirm that the QtWebEngine GPU acceleration was still disabled - it was.

This seems to make it less likely that QtWebEngine GPU acceleration is behind the problem. The jury is still out on W10 Virtual desktop mechanism versus KVM switching between active computers.

If it goes again this afternoon, I'll try another of Kevin's suggestions to restore the menubar.