Date picker: too easy to dismiss and lose changes
When changing the date of an issue in the calendar, it's too easy to dismiss the dialog by de-focusing the picker. This will discard the date you have selected (i.e. clicking outside the picker and clicking "cancel" are equivalent). Because very many date pickers forms like this update immediately, without an Apply button, it's extremely common to then lose the new date.
Also the apply button is not obviously part of the picker as it's below the calendar area, whereas the affordance to open the picker is above the dialog and the dialog border is very light so the "apply/cancel" toolbar fades into the background dialog:
I suggest that either the picker becomes "update on select" (like, say, the ones on Jira) or do not allow to dismiss the dialog on defocus if there are changes (i.e. make the user deliberately select "cancel" or "accept").