The correct solution would probably be to upgrade the version of bootstrap and bootstrap-datepicker that we use
My understandng of the bug is:
* bootstrap-datepicker finds the first ancestor whose z-index isn’t auto, and sets its own z-index to the found elemenents z-index + 10.
* On Firefox and Chrome, the modal element is returned (its z-index is 1050)
* On Safari, the modal-dialog element is found. Its z-index is auto, but Safari reports this as '0', which makes the z-index of the datepicker 10.
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