When the calendar widget is used on a field that is displayed in a modal, the widget displays behind the modal so you can't see it. None of the fields that use a calendar widget is currently displayed on a modal so this has not been an issue. I encountered the problem when extending horizon with a custom panel that uses a modal with a calendar widget on it.
To recreate, the datepicker must be initialized on modal init, and a date field needs to be added to a modal form. For example I added the following fields to the Admin -> Create Network form. The datepicker init code keys off the start and end field IDs so it's easiest to just use those names.
start = forms.DateField(label=_("Start date"), required=False,
input_formats=("%Y-%m-%d",),
widget=forms.DateInput(attrs={
'data-date-format': 'yyyy-mm-dd'}))
end = forms.DateField(label=_("End date"), required=True,
input_formats=("%Y-%m-%d",),
widget=forms.DateInput(attrs={
'data-date-format': 'yyyy-mm-dd'}))
Initialize the datepicker on modal init somewhere in the javascript:
horizon.modals.addModalInitFunction(horizon.forms.datepicker);
When you open the Create Network form and click in the Start date or End date fields you can see the calendar partly showing underneath the modal.
Fix proposed to branch: master
Review: https:/