Tray icon works, but no main interface when using discrete GPU

**Distribution name and version where applicable**
Debian testing

**Flatpak info**
```
Flatpak 1.14.4

$ flatpak --gl-drivers
default
host
```

**Problem description**
Since a month or so, Steam GUI doesn't appear on screen anymore when I lunch the app using "Discrete Graphics Card". However, the tray icon works as expected.

Everything works fine if I start Steam using "Integrated Graphics Card" instead.

**Does this issue reproduce with native Steam**
Have not tested

Start by reproducing the issue on Debian testing with Flatpak 1.14.4, launching Steam with the discrete graphics card and comparing it with the integrated-card launch. Check whether the tray icon appears without the main window, and test native Steam as requested. Done means the main interface opens reliably with the discrete GPU, or the failure is narrowed to a documented environment limitation.

Written by the indexing model from the issue text.