Further investigation indicates that the behaviour is actually quite bizarre. If I create a new header file in a console project and start adding C++-style comments (but nothing else - no actual code), then the first word that can cause CC to fire will do so. So, if I enter;
then the code completion list pops up for "this", and for any other completable words as type them. If I enter a newline, then it seems the behaviour goes back to normal, and no CC takes place in the comment. So it looks like the code for IsComment() may be a bit buggy.