As mentioned in #193:

I have a PR forthcoming that should improve scaling on Windows (and macOS), however, on both Fedora 29 and Ubuntu 19.04, I'm still seeing scaling-related issues under GNOME: Qt5's AA_EnableHighDpiScaling/AA_UseHighDpiPixmaps attributes seemingly do not respond to GNOME's display "Scale" setting (at least insofar as the sizes of windows are concerned), while I'm reading reports from other Qt-based projects that are facing similar issues -- seemingly related to Wayland... I can confirm, at least, that Qt 5.12 is intentionally ignoring the Wayland plugin, as I see the following warning in the terminal:

Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome. Use >QT_QPA_PLATFORM=wayland to run on Wayland anyway.

While running with QT_QPA_PLATFORM=wayland results in a core dump:

qt.qpa.plugin: Could not load the Qt platform plugin "wayland" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, webgl, xcb.

Aborted (core dumped)

In addition, while setting the AA_EnableHighDpiScaling/AA_UseHighDpiPixmaps attributes on Qubes-OS, I observe that the window-sizes always double, even on lower-resolution (1080p) displays, while font-sizes never do.