Created attachment 329849 [details]
If threre's a html element with a background underneath the canvas, and blending is enabled in the context, composition for some reason goes "additive". Here's a test page: https://codepen.io/dmikis/full/ZvQPBG/.
Created attachment 329850 [details]
Actual result in Safari 11.0.3
Created attachment 329851 [details]
Code of the test page
I forgot to mention that the problem reproduces only on discrete GPUs.
We can reproduce. We've forwarded on to our compositing team.
We've got more complaint from users, and it seems that on some configuration antialiasing doesn't affect the problem (i.e., if antialias is set, rendered result is still "blended" with CSS background). In particular, on macOS 10.11 and 10.13 the problem occurs.
The test page no longer works (stv.maps.yandex.net now gives 404 errors).
Also, this should have been fixed in macOS Mojave. Can you test and report back?
this seems fixed to me.
Tested on MacOS 10.14.5 on 2014 MBP and on iOS 12.3.1 iPhoneX
Mark as resolved?