Back when calendar was an add-on the calendar menus and menu items ("Events and Tasks" etc.) did not appear in the menu bar or app menu of single message windows. Now they do but they shouldn't (and they do not work there anyway).
Discovered while working on bug 1619732. Caused by the transition from overlays to inlining and pre-processor includes in bug 1608610.
One possible fix (somewhat ugly but consistent) would be to break the menu item code (e.g. mainNavigationToolbox.inc.xhtml) into a lot of separate include files, then they could be re-assembled for one window type without the calendar parts and for another window type with the calendar parts.
Or better yet, if it works, define a pre-processor flag and then use that in a pre-processor conditional (#ifdef) to leave out the calendar parts in one place but include them in another. This seems promising.
Finally, we could dynamically insert the menu items where needed using JS as explored in bug 1597596. But that is inconsistent with the way the rest of the UI is implemented so not ideal.