QT5 ?
Hmm, maybe. I had it working on my U18.04 install earlier this year, so I made a small test app. Just to see if its a practical solution. It built OK on my Ubuntu18.04 with Lazarus Fixes. Running it however on a number of systems -
* U18.04 - Failed "undefined symbol: QTextEdit_setPlaceholderText". Maybe some recent updates broke it.
* U19.10 - OK if I first install libQt5Pas - that was easy. And a much smaller install than GTK2. Promising.
* Fedora 30 Plasma - Ok if I install qt5pas. Same with gnome but no TrayIcon.
* Mageia - Install a lib64qt5pas1 but test app failed "undefined symbol: QTextEdit_setPlaceholderText"
* Debian 10.1 - Install libqt5pas1 - works OK.
So, works with no more than installing libqt5pas on 60% of what I consider the "big five". But you need to know the name of the pas library, it varies. Sigh. But does not work with 40% and thats a big lump to have to chase down if you are supporting a number of end users. GTK2 delivers 100% but on some of those, now required a largish dependencies download. GTK3 delivers on 100% but is still quite buggy.
Sorry sash, my vote is still "lets get GTK3 up to scratch" !
EDIT: No, I made a mistake, the Qt5 libraries are not pre installed on the mainstream Ubuntu, Fedora and probably Mageia. I use Virtual Boxes to test but I have each distro setup with multiple Desktops, and that includes KDE/Plasma so the Qt5 libraries have come from there. A manual install uses about 49Meg on Ubuntu 19.10, compare that with 321Meg to put GTK2.0 on the same box. Hmm...
Davo