Removing the cover for this light fixture
Pull the spring-loaded prong away from the dome, and tip the dome below it - then pull the dome away from the other two prongs. Don't push the spring-loaded prong with the dome. 


Simple geometry says that the required distance of movement to get
one prong clear is considerably less than what's need to clear the
ones at 120 degrees from it.
Trigonometry quantifies that to about half, if I'm constructing the
problem correctly in my head.