You can force it by a "Windows Rule". In KDE go to System Settings -> Window Behaviour -> Window Rules.
Then create a new rule. WindowClass=steam and in Size&Position check activity and set it to force.
This is only a workaround and the bug still need to be taken care of, but it is better than nothing.
It should be noted that the same thing happens for every application I've tried to date that doesn't use KDE's window decorations (most prominently Chromium).
To me, this implies a Bug in KDE rather than in steam, but I don't know enough about the code to be 100% certain.
Issue still exists with Steam and KDE Plasma 5.14. Notably no longer affects Chromium.
Using kernel 4.18.12 on Arch. Suggested work-around still functions, however.
Still an issue. Can confirm with Plasma 5.15.
This is not an issue with Plasma nor Steam. This just comes from having such an open surface like X. Nobody can do anything about it and no one is to blame either since there is no standard and there shouldn't be either.
If an application does not use native decoration kwin cant do anything about it.
Use (any) Breeze (fork) then force decorations onto the applications (as explained earlier here) and then disable the decorations through the breeze configurations.
Works fine for every application I tested yet (which includes Steam and Wine).
Btw I am using this script (
https://store.kde.org/p/1235093/ ) for forcing kwins decorations. I just edit the code directly when adding new applications since it is easier and faster to do so than Plasmas own dialog.
Nothing extracted yet.
As I posted here; http://steamcommunity.com/app/221410/discussions/0/864956554526511695/?tscn=1357734304
In KDE there's a feature called 'activities', see here http://userbase.kde.org/Plasma/en#Activities . When steam is started it, it shows up in all activities, on the same virtual desktop (i.e it always shows up on desktop 3 when I start it on desktop 3, but for all activities), while it should only show up in one activity at once.
This also happens in the latest update from this morning.
Specs'n'stuff:
-Lenovo thinkpad E130 laptop, i3 2367M with HD3000 and 8GB RAM.
-Sabayon Linux, latest version, using KDE 4.9.4 and kernel 3.7.0
Note that from what I heard a lot of bugs will be swatted in the upcoming KDE 4.10 release, I don't know what subsystem causes the bug, so if it's KDE it might get fixed in that release.
Keep up the great work! You guys are making history here!