Here's the only thing I've discovered so far. On the test project, when the calendar opens it is about 5 cm wide (by holding a ruler up to my screen). In my actual project where the problem is occurring, the calendar opens to about 6 cm wide. Yes, that's right - the problem happens when the calendar is too wide. This seems counter-intuitive but I've checked myself twice.
Does this give you any insight as to what might be happening?
Does this give you any insight as to what might be happening?