On http://everlastwindowsanddoors.ca/contact-us/ just below the "Contact us" banner there is a bunch of text and the contact form. Without WR this area has a whitish background but with WR enabled the whitish background is not there and instead the page's fixed background image is visible, making the text hard to read.

I reduced this page to just a scrolled clip (using the "clip" property) on a fixed element. This testcase looks very similar to a reftest I remember writing before. So are scrolled clips on fixed elements just not working with WebRender at the moment?

I updated the reduced testcase to add a "transform: translateZ(0px)" on the fixed-pos item, which is present in the original page. This affects the display list because it introduces an extra nsDisplayTransform wrapper that sits inside the FixedPosition but outside the Background. I had a patch that fixed the first version of the reduced testcase but failed because of the extra nsDisplayTransform, so this test case is more representative of the real page.

Taking this even though it's a P3 [wr-reserve] because I ended up fixing it as part of my investigation into bug 1408792.

^ This patch applies on top of bug 1373802. Try push that includes both patches: https://treeherder.mozilla.org/#/jobs?repo=try&revision=5bbe693d58de7972add965cdda0b55dc092f43f9

Comment on attachment 8922865 [details] Bug 1411249 - Handle yet more clipping cases. https://reviewboard.mozilla.org/r/194008/#review199100

Pushed by kgupta@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/aa6f720f86f0 Handle yet more clipping cases. r=mstange