Making a working Qt/X11 program work on Windows, on the other hand -- that could be painful. I maintain a Qt application for Linux/macOS/Windows, with in total

Making a working Qt/X11 program work on Windows, on the other hand -- that could be painful. I maintain a Qt application for Linux/macOS/Windows, with in total ~70kloc the last time I looked. It started as a Linux application, but porting to Windows was not a lot of effort and the amount of platform specific code is very small. The painful part on Windows are some non-Qt dependencies, such as libxml2, libxslt, DB XML, etc. The port to macOS required much more platform-specific code to implement macOS-specific behavior (such as keeping the application running when the last window closes), building working, signed application bundles, etc.