Steps to reproduce this bug:
1) Install clean 6.17
2) In Options->IRC->Messages: Check multibyte display, multibyte editbox and utf-8 display. (I installed on a Western European system so the multibyte options weren't checked by default).
3) Connect to a server, join a channel.
4) Type /font in the channel window. Select MS Gothic, MS PGothic or MS UI Gothic. Choose "Japanese" as the script. Set UTF-8 to "display and encode".
5) Enter a long line of Japanese text. Pretty much anything will do. (For example copy something from the Japanese Wikipedia)
6) Slowly resize the window horizontally. The line will corrupt at some widths and look fine at others.
