Scroll to the selected item of ThumbnailScrollBar.

def scroll_to_item(self, index):
        """Scroll to the selected item of ThumbnailScrollBar."""
        spacing_between_items = self.scene.verticalSpacing()
        height_view = self.scrollarea.viewport().height()
        height_item = self.scene.itemAt(index).sizeHint().height()
        height_view_excluding_item = max(0, height_view - height_item)

        height_of_top_items = spacing_between_items
        for i in range(index):
            item = self.scene.itemAt(i)
            height_of_top_items += item.sizeHint().height()
            height_of_top_items += spacing_between_items

        pos_scroll = height_of_top_items - height_view_excluding_item // 2

        vsb = self.scrollarea.verticalScrollBar()
        vsb.setValue(pos_scroll)