My question is, how did you make it run exactly the same on all platforms (Windows, macOS, and Linux). No other cross-platform software works as seamless like this.
We have our own fully custom UI toolkit with a platform abstraction layer. Since we're the only users of this toolkit we have the flexibility to quickly make sweeping changes that can go all the way from the platform, through the toolkit and to the application code. We also generally make platform integration a priority - which you can see from about ¼ of the changelog being platform-specific fixes. This would probably make a good blogpost some day :)