Hello!
When setting a tileLoadProgressEvent and adding a label once all the imagery tiles are loaded, the canvas "blinks" (turns black for a moment): http://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=cf1d71eefb161f3f3546ad4e3d488423
It looks like this issue was introduced in the 1.33 update, it worked fine before.
Thanks in advance!