Thanks for your bug report. I have not been able to reproduce this issue here so far.
I checked the latest version of mIRC under Windows XP, 7, 10, 11, and Wine under Linux, and the buttons are working correctly for me here.
Can you confirm which versions of Windows you are using where this happens?
As far as I know, this the first time this issue has been reported since the feature was added in 2021. Is it possible that this may be related to something specific about your Windows configuration?
