So I really see nothing in the log that is actually the cause of a flicker besides using gtk and adwaita (which is today a part of gtk).
gtk has had flickering problem potential up until this year (still marking duplicates) for a long time now https://bugs.eclipse.org/bugs/show_bug.cgi?id=517671 so you can see if that solution is of application to the issue here.
Clarification - On one hand, the application depends on the OS and other bits to display correctly which is how I lean on this. But, I can see how making sure the dependencies are there or possible coding errors would suggest its at fault with Steam. However, I'm quite conflicted on the latter point with making it at least 50% of the greater chance. To me its < 50% here.
Hello @tapwag, are you still experiencing this issue on an up to date system?
Closing pending feedback.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
Steam login window should be displayed okay.

Steps for reproducing this issue:
SteamConsoleOutput.txt