As at @deviantollam says quote:-
attempting to click on the checkbox is really challenging especially on high resolution displays".
If you’re off by even a pixel, above or below, eM “Client treats this as though you clicked on the “name” of the calendar”.
I agree it’s very annoying when this happens.
To fix this problem "maybe eg:-
Decrease the hotspot clickable area around the checkbox so that it “does not go past the checkbox outline” to then avoid accidental clicking the small space in between the checkbox and the name of the calendar.
Also maybe decrease the hotspot area “around the Calendar name itself” to avoid clicking the small area to the left of Calendar name.
Apart from that I do also like the idea that @lelabo3d had of activating several calendars at the same time with only one check box.