I'm failing to reproduce this. Perhaps you got some certain setting set? Also, what os are you using?
Can you reproduce this, if so, how?
I can reproduce it at any time, just tick to show the treebar.
I'm on Windows XP SP2.
Are you sure you don't have any DLL files loaded that might be causing this? I've never seen this before except when an older (pre-6.2) DLL is "docking" or otherwise messing around with mIRC's children controls (treebar, switchbar, toolbar, MDI). These older DLL files have not been programmed to handle the treebar control or multiple line toolbars, since these abilites are new to mIRC 6.2.
Probably have, since noone else seem to have this problem. I don't know what dll that would be though. The only dll I have is mir.dll but that's just called for when I use my playing now script.