It just appears as a minimized or background window so the way I notice it is Dorico becoming unresponsive. When I click OK Dorico returns to working normally so it doesn’t seem to be something seriously wrong. What is causing this? I can’t say for sure, but I think it started when (or in close proximity to) I installed some choir sound libraries I wanted to try out.
please have a look at this knowledge-base article.
Though that article mentions Cubase/Nuendo as the host application, but since Dorico’s audio engine is based on the Cubase audio engine, that knowledge-base article is applicable likewise in the Dorico context. And also in the Dorico preferences you can toggle Win RT MIDI (Preferences dialog > Play tab > Advanced Options).
I think I found what the problem was. I have been using loopmidi and midi-ox to be able to have the stand-alone halion generate sound from the midi keyboard and not dorico. There was a windows update that included an update to the midi handling so now windows supports using the same midi keyboard as input to several applications simultaneously. Apparently the updated windows midi driver did not work well with the third party midi routing.