brundlefly
Have you rebooted since this started? Sounds like your audio driver/firmware has gotten into a bad state.

Yes. I have done all the common things expected from an experienced user.
I have now ruled out the audio interface: I can change to a completely different audio interface, with windows 10 specific drivers and the stuck-on record button still occurs on newer projects and newly created blank projects.
The problem does not occur on projects that were started several months ago or earlier.
 
I wonder if something outside of Sonar caused this? Maybe a windows update? Maybe a flakey C++ library installed from a third-party plug-in?

Problem solved.
"Alpha Track" was set to "HUI" instead of "Native".
 
Thank goodness it was a stupid problem not related to Sonar.
Thanks everybody for your help.
 

As Dok mentioned disabling arm record during playback solves it for me also.
 

teego
As Dok mentioned disabling arm record during playback solves it for me also.

Cool, Teego. I have seen that one posted about disabling arm recording and I'm sure the bakers are working on it.
 
And I hope this helps someone else: having the wrong setting on your Control Surface causes weird problems.
Platinum runs like a champ now!