Since ugrading to LrC v10.0 (Windows 10), when I open the Develop module the image appears in a small thumbnail and not in the normal window. The thumbnail is not moveable, resizable or deleteable. All develop activities seem to work as normal but on the thumbnail. I have disabled the GPU which makes no difference. Screenshot shows what I'm trying to describe.
I think I might be getting close to a solution. I'm using LrC on a laptop with very high resolution screen, so I'd forced Lr to override the high DPI settings in order to fix a problem with absurdly large text on the Libraray/Develop/.. etc panel. If I switch this override from 'System (Enhanced)' to 'System', it fixes the Develop issue, at the price of slightly fuzzy text. Screenshot shows the detail.