Despite the tickets being sold out, the stadium was still overflowing with hopeful supporters.

overflowing 是动词 overflow 的现在分词形式。overflow 由前缀 over-（表示"超过"）和动词 flow（流动）组成，字面意思是"流出来、溢出"。在句中用作比喻，表示"充满、挤满"的意思。