fully reproducible for me on multiple machines, browsers, and operating systems
1. Start on the site home page
2. Click an event on the home page calendar
3. Click the title of the short display of the event to expand it on the home page
4. Click the "Full details" link to jump to the event's page
5. Now click "Home" at the top of the left navigation bar to return to the home page
6. Click "Full details" again in the event display under the Home page calendar
You will jump back to the event's page, but now the "Back to calendar"
button is nonfunctional and the calendar is inaccessible except through the reset arrows.