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.