I finally figured it out. The problem seems to be the use of the macro IT_CHAR_HAS_CATEGORY, which is modeled after IT_DISPLAYING_WHITESPACE. IT_CHAR_HAS_CATEGORY uses a series of ||. I changed it to a function that uses if … else if … else (which IMO should be the correct way to implement it anyway), and the problem went away.

Please have a look at the patch and see if it’s ok. If you think it’s good I can then update NEWS and the manual and submit a bug report. wrap.txt is the file I used to test word wrapping. To enable the full feature, set cjk-word-wrap to t and load kinsoku.el.

Yuan