Steam fails to run with nvidia drivers: "SteamUpdateUI: An X Error occurred"

I'm on Debian buster, running Flatpak 1.8.2 from buster-backports and the proprietary nvidia driver version 450.66 from buster-backports. After the initial progress dialog that reads "Pins potentially out-of-date, rebuilding...", the following error is printed to the console and the process hangs indefinitely.
```
SteamUpdateUI: An X Error occurred
X Error of failed request: BadValue (integer parameter out of range for operation)
```

I have rebooted and rerun `flatpak update` as suggested on #563, but that made no difference. I have encountered similar X errors from Steam and only had to rerun `flatpak update` to update the nvidia "platform", but that is not working now.

Steam from the Debian package in the "non-free" package repository runs fine, even under firejail, so it does not seem to be missing libraries on my system.

The entire session output is as follows:
```
$ flatpak run com.valvesoftware.Steam
https://github.com/flathub/com.valvesoftware.Steam/wiki/Frequently-asked-questions
Relocating /home/aidan/.var/app/com.valvesoftware.Steam/config/user-dirs.dirs to .config/user-dirs.dirs
Overriding TZ to Pacific/Auckland
Setting up Steam content in /home/aidan/.local/share/Steam
Running Steam on org.freedesktop.platform 20.08.0 64-bit
STEAM_RUNTIME is enabled automatically
Pins potentially out-of-date, rebuilding...
Can't find 'steam-runtime-check-requirements', continuing anyway
/home/aidan/.local/share/Steam/ubuntu12_32/steam
[2020-10-06 20:11:58] Startup - updater built Jul 28 2020 22:37:33
ILocalize::AddFile() failed to load file "public/steambootstrapper_english.txt".
Installing breakpad exception handler for appid(steam)/version(0)
SteamUpdateUI: An X Error occurred
X Error of failed request: BadValue (integer parameter out of range for operation)
```

Reproduce `flatpak run com.valvesoftware.Steam` on Debian buster with Flatpak 1.8.2 and NVIDIA driver 450.66, and compare the session output shown in the report. Investigate the `SteamUpdateUI: An X Error occurred` and `BadValue` failure; done means Steam starts without hanging after the update dialog.

Written by the indexing model from the issue text.