I am on a fairly underpowered machine, but Designer has worked for me in the past. I just updated to 1.8.5 from 1.8.4 and the application opens fine, and I can choose a canvas for a new document, and it opens. Once the document opens nothing works but the cursor. I have tried keyboard whortcuts and clicking on every icon and hotspot. There is no response from Affinity at all. It was working fine as 1.8.4 15 minutes before I updated it.
I had very few addons: a couple of brush packs and several fonts/assets but that was it. I can't even take a screen shot. It's like it's not there, but it is stuck on my screen.
Affinity_Designer_2020-08-24_14-46-46.mp4