TL;DR: if you have Steam running for more than a ~day or so, you will run out of window handles so you won't be able to open any new graphical application/window until you restart Steam.
Using Steam Chat appears to make the issue worse (it happens earlier).
This has been documented under https://github.com/ValveSoftware/steam-for-linux/issues/9094 but for some reason that issue has been closed.
I personally just restart Steam every day but if someone else encounters this issue and doesn't know why their windows are not opening, this is why :)
I am using KDE/Wayland but I've observed this under X11 too.