fix(DatePicker): support focus into a day with min/max (#4793)

This change allows user to focus a date into down-arrow key when
min/max dates are enabled. When min/max dates are enabled, there can be
no selected date in the calendar dropdown or calendar dropdown may not
show today, so we need to look for another date to focus on. This
change uses the first focusable date.

Fixes #4207.

packages/react/src/components/DatePicker/DatePicker.js

@@ -412,6 +412,7 @@ export default class DatePicker extends Component {
           (
             cal.selectedDateElem ||
             cal.todayDateElem ||
+            cal.calendarContainer.querySelector('.flatpickr-day[tabindex]') ||
             cal.calendarContainer
           ).focus();
         }
@@ -429,6 +430,7 @@ export default class DatePicker extends Component {
           (
             cal.selectedDateElem ||
             cal.todayDateElem ||
+            cal.calendarContainer.querySelector('.flatpickr-day[tabindex]') ||
             cal.calendarContainer
           ).focus();
         }