This works perfectly for me, I don't think any changes were made to 6.15 in this respect. I've tried several combinations and cannot do what you have done.
What OS are you using? Are you using any DLLs (I've heard there's an issue with some docking DLL)?
Install a fresh mIRC into a new directory and see if the problem still occurs.
I tested this under 6.15, XP Pro.
Regards,
