Thanks for your bug report. I was able to reproduce this issue, although for different string lengths to yours. I found an interesting discussion on a similar-sounding issue here. After some testing, it does seem to depend on how long it takes for ExtTextOut to display the text. When using fonts that take more time to display, I see fewer lines, and switching to a faster font, I see more lines. Unfortunately, the solution described on stackoverflow cannot be used by mIRC's display routines, which are more involved, especially when handling Unicode. I have added this to my to-do list for now.