According to your screenshots the viewport is sharp. I will explain what happens on example of "Retina" displays under macOS. "3DCoat" supports HiDPI (Retina) displays by rendering more pixels in the viewport. To render user interface elements and text on HiDPI displays "3DCoat" upscales them to maintain the same physical size of user interface on Retina (144 ppi) and non-Retina (72 ppi) displays. I have attached screenshots to explain this. The first screenshot is made on my non-Retina external display (72 ppi). The viewport and user interface are equally sharp because they are rendered without scaling with the same 72 pixels per inch resolution. The second screenshot is made on Retina display of my MacBook Pro (144 ppi). The viewport is sharp (144 pixels/inch). User interface elements are upscaled by two to match those 144 pixels/inch of the viewport. Otherwise the elements would be small. They are blurry because user elements and font textures in "3DCoat" have resolution 72 pixels/inch. Note that "3DCoat" doesn't have any special code to support HiDPI under Windows. "3DCoat" doesn't upscales its user interface under Windows in any case. That "upscaling" becomes from Windows system setting "Change the size of text, apps, and other items" which effects in the same way as I have explained above.