>> I understand it's important in general, but the question is for this
>> specific use of `org-funcall-in-calendar` where all we do (apparently)
>> is to set `cursor-type` which shouldn't require any change to the
>> overlay (nor does it require to `select-window`), or should it?
> No, it should not, and it does not require `select-window'.

OK, changed it to `with-current-buffer`.

I pushed the resulting patch (along with three other patches resulting
from running the tests) to `scratch/org` on `elpa.git`.

You can also find them attached,


        Stefan