I am reviewing my old bug reports, and can confirm this one is still present, at least for the KDE desktop.
Here is a simpler way to trigger it:
Distribution: Arch Linux
Desktop environment: KDE plasma 5.9.5
I also tried this on XFCE, the background window gets reactivated instantly, which is consistent with the expected behavior. I let you judge if this needs to be fixed or not, but I can understand that the priority is rather low.
Nothing extracted yet.
Steps to reproduce :
Interpretation :
It seems that The steam warning message is unable to bring itself back to foreground.
The error sound playback should at least be stopped, because it is really annoying.
It may also be a problem with others error dialogs, I didn't tried.
System Info :
Distribution : Slackware linux 14.0 64.
Window manager : KDE 4.10.3 (trying with others desktop managers right now, I will edit this issue)
EDIT : Seems not to happen with Xfce. Maybe an issue related to effects? I am running out of games to update...
Desktop effects enabled (OpenGL) with KDE.
Latest steam beta build at the time of writing (Wednesday, 15th may 19:45 GMT)