... is this a comment from 1997 ? Qt is licensed under both GPLv3 and LGPLv3 - and Trolltech as a company hasn't existed for more than a decade...
And trolltech is very alive and publicly traded in fact. It has just been renamed to Qt company: https://en.wikipedia.org/wiki/The_Qt_Company
GTK+ and wxWidgets are other viable and popular choices for cross-platform desktop GUIs.
While Flutter doesn't work yet for desktop or web, ReactXP already works for these environments, and is used in the new Skype for Web.
Windows support is done and targets W10, Xbox, and Windows Mixed Reality. MacOS support is experimental, but mostly working. Linux currently needs an Electron wrapper.
There's not much of an ecosystem for multiplatform yet for desktop projects at all. For macOS you have a way of "making a framework you can use" and that's about it.
There is a lot of promise, though, in the mobile space: https://touchlab.co/touchlab-square-kotlin-multiplatform-col...
My sense is that as the mobile multiplatform space gets ironed out, the desktop space will benefit.
Though generally speaking, Kotlin/Native has a _much_ lower level of "institutional" risk, since the Kotlin approach doesn't subvert the control Apple or Microsoft have over their platforms. Flutter, on the other hand, seems to abstract it away.
Its also pretty slow compared with other native solutions.
If you want a debugger you need CLion, though. And the compiler is really, really slow – but they seem to be working on benchmarking things recently, so presumably it will be improved.
Check the IDE: (https://www.lazarus-ide.org/) and the cross-platform apps built with it: http://wiki.freepascal.org/Lazarus_Application_Gallery