I have some additional info for this glitch.
It happens every time with "-silent" autostart.

Steam creates a 64x24 transparent window fixed to top left corner of the screen when started with autostart.
This pseudowindow is somehow corrupt, because it cannot be moved, can't be killed with xkill, and it also blocks access to things under it. It is fixed to the corner of the viewport permanently until steam is killed and started manually.

It also persists in fullscreen games:
ironically, your own launcher ruins gameplay in the games you sell!

I am experiencing this in KDE 4.10.1.

I reported it as originally a bug to KDE bug tracker, and they told me to use xprop and xwininfo on this weird artifact. So I did that, and surprisingly, the glitch was labeled "Steam"!

You can read the bug report I made here: https://bugs.kde.org/show_bug.cgi?id=316565
There are also outputs of xprop and xwininfo attached there.

[bump] guys I think this ought to get some attention. You do something really strange with the x windows.

The steam launcher is docked, yet:

ondra@Penguin:~$ xwininfo -root -tree | grep -i steam
0x42000d4 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000d2 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000d0 "Správce snímků": ("Steam" "Steam") 64x24+0+0 +0+0
0x42000ce (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000cc (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000ca (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000c8 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000c6 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000c4 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000c2 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x42000c0 "Nastavení": ("Steam" "Steam") 540x586+690+247 +690+247
0x4200075 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200073 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200071 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420006f (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420006d (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420006b (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200069 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200067 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200065 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200063 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200061 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420005f (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420005d (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420005b (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200059 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200057 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200055 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200053 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200051 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420004f (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420004d (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420004b (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200049 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200047 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200045 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200043 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200041 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420003f "Servery": ("Steam" "Steam") 64x24+0+0 +0+0
0x420003d (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420003b (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200039 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200037 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200035 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200033 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200031 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420002f (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420002d (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420002b (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200029 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200027 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200025 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200023 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200021 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420001f "Steam": ("Steam" "Steam") 64x24+0+0 +0+0
0x420001d (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420001b (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200014 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200012 "Přátelé – MightyPork": ("Steam" "Steam") 64x24+0+0 +0+0
0x420000c (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x420000a "Steam": ("Steam" "Steam") 64x24+0+0 +0+0
0x4200008 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4200005 (has no name): ("Steam" "Steam") 64x24+0+0 +0+0
0x4400001 "steam": ("steam" "Steam") 10x10+10+10 +10+10

I am not extremely experienced in working with X windows, but I've never seen such a huge collection of junk windows. In this case, the "0x4200075" is the window which causes that ugly artifact i mentioned in last post.

(the corner artifact does not refresh, so the pixels it shows are from some app which was at that position when the window manager was started)

I think you forgot to unmap this window, whatever it is used for.
And if I manually unmap it with "xdotool", then steam thinks my mouse is two inches higher than it really is - which is really weird.

The title of this bug should be changed to something more appropriate. I am sure the "stealing focus" is caused by this pseudowindow artifact you make there.

Confirming that this bug still happens, and only happens when Steam is launched silently.