Spotted in the wild at https://magic.wizards.com/en/articles/archive/making-magic/rivals-no-other-part-1-2018-01-08. Attached reduced testcase shows how an inner div with style "transform: rotateY(0)" enclosed by a div with style "perspective: 1000px" breaks Z order. Expected results: Both inner divs (red) should appear under the black partly-transparent cover. Actual results: The first div appears on top of the cover. When Webrender is turned off, expected results are observed.

mozregression --good 2017-10-10 --bad 2018-01-10 --pref layers.acceleration.force-enabled:true gfx.webrender.enabled:true gfx.webrendest.enabled:true gfx.webrender.layers-free:true gfx.webrender.blob-images:true image.mem.shared:true layout.display-list.retain:false startup.homepage_welcome_url:"https://bugzilla.mozilla.org/attachment.cgi?id=8944955" > 10:02.99 INFO: Last good revision: ef9858e9ad38718eea180a62bb5b65561ad57d91 > 10:02.99 INFO: First bad revision: cdf4bb1c6af6e53643d2beb0f10a2ce29207ff5e > 10:02.99 INFO: Pushlog: > https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ef9858e9ad38718eea180a62bb5b65561ad57d91&tochange=cdf4bb1c6af6e53643d2beb0f10a2ce29207ff5e > cdf4bb1c6af6 Kartikaya Gupta — Bug 1417062 - Update for API change in WR PR 2043. r=nical > 72cfd88fe0a0 Kartikaya Gupta — Bug 1417062 - Update reftest annotations for changes in WR PR 2031. r=nical > c80c224d6b0c Kartikaya Gupta — Bug 1417062 - Update webrender to commit d490a74c438d987122c600afca6bb2247ab38637. r=nical The try builds are gone.

I have some of the try builds saved locally. WR @ d8fe5bdc791647ff5b82b82428265cc51783ce4d - good WR @ 54e796882beb0c28fd0fb03164131392abdf3fe8 - bad range is: * 54e79688 Auto merge of #2031 - glennw:pic28, r=kvark * fadae648 Address review comments and fix Gecko test failures. * 0633bb37 Hide StackingContext * 6ca43c34 Introduce PicturePrimitive::Image.

This will be fixed once https://github.com/servo/webrender/pull/2369 lands in Gecko.

try builds from bug 1434723 comment 6: > WR @ c383ef73113d0d5f8634385565ce83a4c84ca944 > https://treeherder.mozilla.org/#/jobs?repo=try&revision=5cd09997d5e0c3ed19d6c3304b79931f86f25b56 mozregression --repo try --launch 5cd09997d5e0c3ed19d6c3304b79931f86f25b56 --pref gfx.webrender.all:true startup.homepage_welcome_url:"https://bugzilla.mozilla.org/attachment.cgi?id=8944955" -> bad https://github.com/servo/webrender/commit/7809537ea48090c4d8e36aff83149f776c5f7bee > WR @ 7809537ea48090c4d8e36aff83149f776c5f7bee > https://treeherder.mozilla.org/#/jobs?repo=try&revision=3535941651e34d664076963b5435b5d60546869d mozregression --repo try --launch 3535941651e34d664076963b5435b5d60546869d --pref gfx.webrender.all:true startup.homepage_welcome_url:"https://bugzilla.mozilla.org/attachment.cgi?id=8944955" -> good, fixed Thanks! :)

Bug 1434723 has resolved this.

This being fixed with a PR - is there a Gecko side reftest that we can add that catches this problem?