After clicking on the "Snooze for" button, the focus remains on the reminder rather than the drop-down. If yor manually move the focus to the drop-down, the tab key doesn't move through the check button and the close button, it simply cycles between the two fill in boxes.

Lightening should work like Sunbird in that the focus should shift to the drop-down and cycle through the fields and the two buttons on the drop-down.