Hi,
I am faced with a curious problem using SS (3.1.2 and 3.1.6) and event_calendar.
When browsing my test web site, whenever I click on the Calendar option menu, the search entry area, in the header, is overwritten, for a second or two, by graphic symbols (windows, É, Movie symbol, star, star, E, star). This occurs even in a freshly installed SS 3.1.6 site, with module event_calendar and fullsearchtext enabled and empty database and only a Calendar page created.
Moreover, in my dev site, I also noticed that by clicking on Calendar, the height of the header is first cut in half, then after a second or so, returns to its normal height, causing a jerk on the screen. This does not happen for other menu options.
While investigating, i found that if remove this line

 <div class="calendar-widget" data-url="/agenda/" ></div>

in the page source code, the jerk problem goes away (but so the calendar Widget !)

Thanks for your help.