It has nothing to do with Windows. Read Khaled's FAQ, he wrote the tooltips:
Answer: Yes, the Toolbar and tooltips, the Tabbed dialogs, the Tree List in the Options dialog, the Switchbar, the Text Display in windows, and one or two other controls, are all custom coded.
The reason for custom coding these controls is that they formed an integral part of the interface, and I wanted to make sure that they worked the same way under all versions of Windows, including Windows 3.1 for the 16bit mIRC.
As I said, since Win3.1 support is gone, I think mIRC should use the Windows tooltips since for us XP/2k/2003 users who have additional tooltip options, these features are disabled in mIRC since mIRC's tooltips don't support it.
