Created attachment 286127 [details] actual rendering opacity, position, mix-blend-mode, filter should only create a new stacking context for its fixed descendants, and not create a containing block. Instead, in Safari this gets created, causing cropping of the fixed descendants http://jsbin.com/coxoye/2/edit?html,output
This is a compositing bug.
Created attachment 311507 [details] Picture of the bug on chrome(left, how it should look like) and on safari (right, with bug)
I was having this bug too. Tested on iOS 10.3 (safari, iPad). Link to demo: https://codepen.io/sofiapoh/pen/xdozJL
Per the spec, *filter* should create a containing block for fixed-pos descendants; see my comment at https://bugs.chromium.org/p/chromium/issues/detail?id=797493#c6 . That said, based on my test at https://dbaron.org/css/test/2018/stacking-context-z-order WebKit isn't creating a containing block for fixed descendants for any of these properties (perhaps comment 1 was indicating that comment 0's statement is too general?).
We don't see an issue with this test case. If you believe there is still an issue, could you please REOPEN this bug with a small test case illustrating the issue?