Hi Chris.
Thank you for the very fast response.
Later in the day I noticed odd behaviour in other GPU accelerated apps. I have a number of them concurrently open throughout the workday and haven't yet isolated the culprit but it seems from a spindump that some system resources were locked that Affinity needs in order to open a file. When the Apple Photos app exhibited similar behaviour it was pretty clear I had a systemic problem. Some candidate bad neighbours are Zoom, Skype, Chrome and Safari Preview.
Short version - after a reboot Affinity is back to working as expected. I was probably a little quick on the trigger filing a bug report before even a reboot, but having been a *nix user for almost 30 years I'm just not used to rebooting very often
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Can't see any way to close this bug - but then I guess we are using forum software for bug tracking. Feel free to delete the thread to reduce noise.