As documentary filmmaker Kiwi Chow walked through a pedestrian tunnel in Hong Kong on a recent day, he spotted a team of cleaners scraping off glue left by illegal ads and scrubbing the walls clean with mops.

It reminded him of the day he became an activist during the pro-democracy protests that galvanised this Chinese-ruled city in 2019. Back then, he had gathered with his young son and some friends to cover another tunnel wall with Post-it notes scribbled with political messages and drawings.

Strangers joined in...