If you have customized your mail toolbar to have Folder Location and/or Mail View widgets they still show when you switch to a Calendar or Tasks tab. Changing those widgets then makes the change to the last mail tab you were in (which is wrong in my opinion).
I wonder whether Bug 536302 is related, > Calendar tab or Tasks tab shows Mail and Newsgroups (not Calendar, not Tasks)
I think this should rather be a Thunderbird bug. The command controller should disable all unrelated elements when switching to an external tab, unless of course the external tab controller re-enables these commands. If the widgets (i.e folder location) should also work in other tabs, then they should auto-switch to the mail tab, in which case they can stay enabled.
Component: Lightning Only → Toolbars and Tabs
Product: Calendar → Thunderbird
QA Contact: lightning → toolbars-tabs
Summary: Lightning should hide Folder Location and/or Mail View widgets when showing Calendar/Tasks Tab → Folder Location and/or Mail View widgets should be disabled or hidden when showing Other Tab Types
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