When trying to select a single Japanese character by highlighting/selecting with the mouse so it may be pasted elsewhere, mIRC will copy nothing. It appears that the routine to place the selected text in the clipboard after the the MB1(MouseButton 1) is released is not invoked.

My settings were:
mIRC 6.2 Options > IRC > Messages:
checkbox: SJIS/JIS conversion unchecked, with the other 5 of the group of 6 checkboxes there checked.
Also the font setting for the channel was:
MS Gothic: Script Japanese: Display and Encode

I'm using the default IME that comes with windows xp pro.