Platform Development • Remove GTK+ in favour of UXP

If feasible, then I do suggest to extend UXP as a cross-platform toolkit for the creation of graphical interfaces.

It already is.
XUL is a UI language allowing you to generate cross-platform applications with a consistent UI.
But you have to understand that that is one layer, that necessarily builds upon other layers (GTK+/WinAPI(DX)/CG/etc.). you can't remove the underlying layers and expect UXP to take on that task.

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