Qt AppImage doesn't scale up

 * Cinnamon version: Cinnamon 5.0.6
 * Distribution - Mint 20.2
 * Graphics hardware: Vega 10 XL/XT [Radeon RX Vega 56/64]. Driver: amdgpu (kernel driver)
 * 64 bit

Issue

When using AppImage with a Qt Gui (maybe the same is happening for GTK?), Qt doesn't scale-up on hidpi screens. Although, I enabled double user interface scale (Hi-DPI) setting. Also tried the experimental feature 'fractional scaling' to 200%, without luck.

In all cases the AppImage GUI doesn't scale up.

Steps to reproduce

1. Linux Mint 20.2, using a 4K display
2. Enable User interface scale: Double (Hi-DPI) or fractional scaling (eg. 200%)
3. Download AppImage. eg duckstation qt: https://github.com/stenzek/duckstation/releases/tag/latest
4. Run the AppImage. It will not scale up.

Expected behaviour

I expect all applications, including appimages to scale up their (Qt) GUI automatically if I enabled GUI scaling in the Cinnamon Display settings.

Other information

Example of running DuckStaton emulator Qt GUI on Linux Mint 20.2. As you can see the icons and fonts do not scale automatically.

Thanks.

Regards,
Melroy

Start by reproducing the issue on Linux Mint 20.2 with a 4K display, Cinnamon 5.0.6, and the DuckStation Qt AppImage using both double and fractional scaling. Inspect Cinnamon's display-scaling behavior for externally launched AppImage applications and compare it with the expected scaling of other applications. Done means Qt AppImages automatically honor the enabled Cinnamon scaling setting.