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).