My choice would be (assuming more than one match detected)
To load the editbox with "%nick $2-" and halt the input event
If the user presses ENTER again it rotates to the next matching nick halting the input event again, and onward cycling back as needed. If the user presses CTRL-ENTER it allowes the line to go out.
My personaly way is different again...
I use F1 key to cycle the $1 ( or F2 to use the last word) of the editbox tell i find the nick i want, i used to use the inbuilt nick completor but the channel i talk in has a bunch of people with the same first 4 letters and im always failing to type them in so never complete the nick using TAB.
Only downside to mine is the cursor position in the editbox is lost and any highlighting at the time
