I have found one bug that happen on my Windows 7. When I start tabbed chat for example 2 conversations then move the first conversation window to the new position then I can see the second conversation window stick to the old position. I need to switch back to that struck conversation window then it seem to be ok.
Windows 7 RTM 7600
Windows Live Messenger 8.5.1018
Messenger Plus! Live 4.82.0.368
Thanks for the report, this will needed to be fixed indeed. Are you using a compatibility mode? also, do you use a skin for Messenger, and is Aero turned on?