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.

We used to talk about Windows/Linux for portability topics (conveniently ignoring BSD and others, I reckon). Somehow, Apple took over much of Linux's mindshare. And not just here. Many of my coworkers are asking for apple laptops. A huge fraction of keynote presenters use an apple laptop.

All that is a bit sad, considering Apple is even worse than Microsoft, freedom wise. They popularised the digital prison, that was originally confined mostly to game consoles. I'll never forgive that, no matter how much they contribute to open source.