Move previews after a resize event

def move_previews(self):
        "Move previews after a resize event"

        # calculate new positions
        min_y = self._calc_preview_ypos()
        for idx, (key, p) in enumerate(self.previews.items()):
            new_dy = min_y[idx] - p.y
            self.previews[key].move_by(0, new_dy)
        self._update_cregion()
        self.show_selected(self._sel_id, self._sel_widget)