Comment by toast0

My understanding is that California has two relevant laws for this discussion:

21453 (a) which prohibits crossing the stop line when the light is red [1]. And 22526 (a) which prohibits entering an intersection when the exit isn't clear. [2]

You have to be able to clear the intersection if you enter it, and you have to enter it on green or yellow (except for turn on red after a stop), but you don't have to clear the intersection before it goes red.

Common practice (which might not be 100% legal) for unprotected lefts on green (where space permits) is for the first car to fully enter the intersection and the second car to roll over the line a bit, then for both vehicles to clear the intersection when opposing traffic stops which may be in yellow or red. The driver that's only a bit in the intersection can make a judgement call and stay slightly encroaching rather than clear the intersection if clearing seems inadvisable because opposing traffic was slow to stop.

[1] https://leginfo.legislature.ca.gov/faces/codes_displaySectio....

[2] https://leginfo.legislature.ca.gov/faces/codes_displaySectio....