This looks to be a system driver bug, but logging here for transparency and future reference. It took me a long while to figure out that the key issue is a conflict during blit between UE4 and the HW video acceleration.

From AnswerHub.
I have seen a few post talking about this visual glitches (white flickerings) on mac but they talk about being something that happens in certain modes of unreal.. i get these all the time, in the preview of 4.4 it was way worse than in the 4.4 release but i still get them all the time... does anybody has a solution for this? or can give an explanation? i have been searching for this and nothing comes out.. is it just me? or is a laptop thing?

i use a macbook pro 2.9GHz intel core i7, 8gb, 16MHz ddr3 OSX 10.9.4 (laptop)

Steps to Reproduce

1. Select the Intel GPU of Mac if required.
2. Run UE4Editor ElementalDemo -game.
3. Play a HW accelerated video.

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Affects Versions4.4
Target Fix4.15
CreatedDec 2, 2014
ResolvedDec 2, 2016
UpdatedApr 27, 2018