Forums Feature Suggestions Autocomplete in editable drop boxes

That's not what he's suggesting.
What he says mIRC should support is something most browsers do, like when typing "www.mir", the browser completes it to "www.mirc.com", but it still keeps the cursor after the 'r' and selects "c.com", so you can keep typing if going to mirc.com isn't your intention.
Your script either doesn't do that or fails (I've tested it).