As far as I can tell, both are common, but “white space” is more common as a synonym to negative space, and “whitespace” is more common to refer to the characte

As far as I can tell, both are common, but “white space” is more common as a synonym to negative space, and “whitespace” is more common to refer to the characters. If so, the “white-space” property controls whitespace, not white space.