That is really strange. So it's narrowed down to text selection in an mIRC text buffer area. Nothing else is impacted. This is in Windows 7, and even after changing the buffer size to a conservative 1000 lines from an already conservative 5000 lines.
I wouldn't call this a clipboard bug. I would call this an owner-drawn GDI selection bug. It smells like a memory leak impacting GDI resources (not being freed up).
Are you running any goofy software that might Hook the mIRC process? Antivirus? Debugging software? AutoHotkey Scripts? Anything that snoops, modifies, adds feature or function to other exterior software -- like a text-to-speech synthesizer or VNC remote desktop server or anything like that? Are you loading any DLLs in mIRC? Theme scripts maybe?
