You can only copy the currently displayed text. To copy text from another page, you must scroll up/down to it and then copy it. If you want to store most of the text you see on a channel, you might want to use the logfile/buffer options in the System Menu.
The use of color in mIRC means that a simple text box cannot be used since text boxes can display only plain text (and they also have other limitations). However, text boxes also have built in cut/copy/paste routines which unfortunately are unavailable to a graphic window. This means that I had to code the mark/copy routine myself. I'm not sure which ran out first, my patience or my programming ability :-)"