I'm surprised no one here mentioned Lazarus [1].

Sure, it is based on a language which some people have aversions against (for unclear reasons), but it in fact does cross-platform GUI development almost flawlessly, and almost completely friction-less (no, or very little, code change to compile on the three desktop platforms).

Try open up a stable version on your platform of choice, drop a button on the form, double click and write some code (with autocompletion) and compile ... and then import the project on another OS and compile ... It's all a pretty mind-blowing experience, just how productive and "Just Works" the whole thing is.

To expand a bit, it compiles to compiled binaries in the range of MB, uses native GUI libs on each platform (GTK or Qt on Linux for example)... and you can now link it with a language like Go if you need some heavy multi-core processing, or need to access stuff that FPC doesn't have good libraries for [2].

It was discussed in depth here on HN just 5 days ago [3]

[1] https://www.lazarus-ide.org/

[2] https://forum.lazarus.freepascal.org/index.php?topic=24948.0

[3] https://news.ycombinator.com/item?id=14973706