Calendar library doesn't pass the component variable to $OUTPUT->pix_icon which makes pix_icon default to only looking in the core 'moodle' component, and not the component in question (i.e. activities).
1. Add an activity (e.g. an Assignment) with a due date
2. Add a calendar block to course page
3. Hover over calendar event - tooltip has broken image (shows alt text)
See screenshot for example.
Expected result: See the activities icon, not alt-text.
Patch that fixes this attached.