brightness changes on zoom

Some users are seeing brightness shifts on zooming in to the globe. It seems to occur on some older macbook pros and macbook airs and can be reproduced on cesiumjs.org "tap to interact".

http://recordit.co/5iKJu68DSB


Start by reproducing the brightness shift while zooming on cesiumjs.org using the “tap to interact” globe, particularly on the older MacBook models described. Use the linked recording and image to compare the visual change, then trace the zoom and rendering path to identify the affected behavior; done means the brightness remains stable during zooming.

Written by the indexing model from the issue text.