skin does not scale for high resolution screens with Qt5

[Be]

[Impact]

The library and preferences scale, but not the custom QWidgets for the skins. See attached screenshot in comment #1.

[Test Case]

 * Start Mixxx on a HiDPI screen and verify if all GUI elements are scaled correctly.

[Regression Potential]

Due to the changed way of building the GUI elements there might be visual regressions. Nothing known though.

[Other Info]

The Cosmic 2.1.3 build is also effected, see: https://bugs.launchpad.net/ubuntu/+source/mixxx/+bug/1804513

[Be (be.ing)]

This is with GNOME Shell 3.16 with Wayland on Fedora 27.

[Be (be.ing)]

Mixxx needs to set the Qt::AA_EnableHighDpiScaling property for the application to enable HiDPI scaling with Qt5. This can be enabled without recompiling by setting the QT_AUTO_SCREEN_SCALE_FACTOR environment variable to 1. It is confusing to figure out where the issues are, but this only works reliably for me with KDE on X.org. I had it working with GNOME on Wayland, then I messed with some settings more, and then it didn't work. Regardless, it seems that the issue is with Qt, Wayland, X, XWayland, GNOME, or KDE, but not Mixxx. All that needs to change in Mixxx is setting Qt::AA_EnableHighDpiScaling.

I cannot get the QT_SCALE_FACTOR environment variable to work in any circumstance with Mixxx or any other Qt application.

http://doc.qt.io/qt-5/highdpi.html

[Be (be.ing)]

https://github.com/mixxxdj/mixxx/pull/1521

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/9104