When using -webkit-mask-image on an element that uses a 3D CSS transform the mask is incorrectly scaled to fit inside the viewport in Chrome. Safari works as expected.
Created attachment 152734 [details]
In the test case, the red circle should be translateZ, not translateXZ(0)
Created attachment 153065 [details]
Updating as described above.
I tried this on a 2nd older mac which had hardware accelerated graphics disabled and the issue disappeared. Forcing hardware acceleration via about:flags triggered the bug.