What chinese territory continued its common law system, even after it reverted to chinese rule in 1997?

The answer is Hong Kong. They were under British rule at the time, before it turned to the Chinese. However, there has been protests (recently this year) against the extradition bill that was proposed in that country.

Answer: the treaty of Nanking in 1842 ceded Hong Kong to the British. Their big ships and military might meant China had little choice at the end of the first opium war. It was given to them in perpetuity. It was this, the New Territories, that in 1898 the British pledged to give back in 1997.