As Qt is a GUI library (among other things) I would guess that Paint Pro is written entirely using Qt.
It all just worked.
Their licensing got really goofy for a while, and yocto (which unfortunately I became a “SME” of at work) made cloning and using anything after 5.15.2 very obnoxious. They needed to figure out how to pull a RedHat and get paid support, but this was right around the time electron (at least for a while) rendered native apps as second-class citizens.
The bindings to other languages are neat, and the signal/slot idea is also neat. The whole moc part isn’t great, but I understand why, and it has very rarely caused me any real issues.