I have installed HeidiSQL from Windows Store and it doesn't have the HIGH DPI compat option into the manifest. I can download the executable versión from this page and set the DPI Compatibility mode right clicking the exe and setting High DPI to System (enchanced). Doing this renders the app good and Sharp in my 4K display. By default the app is pixelated on high dpi displays. Doint this is essential.
The thing is that setting could be configured as the default option including three lines into the app manifest and it doesn't affect people who doesn't have a high dpi display. Please, search into the internet this "Improving the high-DPI experience in GDI based Desktop Apps" and the first result will be a Windows Blog article explaining this (sorry, i can't post links)
Go directly to Updating your GDI app to use GDI Scaling to check the manifest. Doing this to the Windows Store app will solve issues for high dpi users!