APEX 5.0.3.00.03 (but 5.1 behaves the same as far as I can see).
I changed the template type of a standard APEX 5 calendar to be 'collabsible', then set the template option to default collapsed.
After the page has rendered, the user clicks on the region to expand it. The calendar header appears but there's no content. If you click to change the month or the view type, the calendar content is then displayed as normal.
I can sort of guess why it isn't working (the calendar perhaps thinks it has a zero size area to draw the content in when the page is initially rendered?) but it would be nice if there was a simple way to make it display properly when the region is first expanded. There was a thread on similar behavior of interactive reports here: IR column headings are not displaying in collapsible region in oracle apex but I don't think it applies in this case.
If anyone has any ideas I'm a-listenin' :-)
Cheers.