Scaling issues on high DPI displays

Edit: tresf, added checklist and workaround:

• Windows: - Partially working via d2e50df3ce2f8a56fd1750cc077eff3e76a6867d.
• Linux: - Implemented via 8a39302571c96e130dd6db1f466011760fd7f1ec reverted. reimplemented bbedfa9. (Qt5.6+)
• MacOS: - Implemented via 8a39302571c96e130dd6db1f466011760fd7f1ec

Unfortunately some HiDPI displays don't scale properly. There's more discussion around this here: https://github.com/LMMS/lmms/pull/4800#issuecomment-461681138

# HiDPI workaround for problematic screens
export QT_SCALE_FACTOR=1
export QT_AUTO_SCREEN_SCALE_FACTOR=0
export QT_SCREEN_SCALE_FACTORS=2

Or permanently (use with caution!)

# HiDPI workaround for problematic screens
echo -e "QT_SCALE_FACTOR=1\nQT_AUTO_SCREEN_SCALE_FACTOR=0\nQT_SCREEN_SCALE_FACTORS=2" | sudo tee -a /etc/environment

This is my first time submitting an issue here, so If I'm doing it incorrectly, please let me know.

I'm on Windows 10 and would like to use LMMS, however, being on a Surface Book with a high DPI display, I can't comfortably use this software without squinting to see the various menu items and text. This application doesn't seem to scale when I scale Windows up. Is this a known issue or is it just me?

Review the Windows checklist item, the referenced commits, and the discussion on pull request #4800 about high-DPI scaling. Reproduce the behavior on a Windows high-DPI display using the provided environment-variable workaround, then update the implementation so the Windows case scales correctly and the checklist reflects the result.