User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.115 Safari/537.36 Steps to reproduce: Go to https://complice.co/ on Android firefox. Sign in as zzztestgoals, password zzz. Click Today at the top if the url isn't already https://complice.co/zzztestgoals/today Click a black button called [Just submit outcomes] (it may require scrolling down the page) A modal should pop up. Scroll the page. Actual results: The bottom half of the modal may start to disappear as the page scrolls, and/or become pixellated. Sometimes (it seems particularly when the keyboard is visible) a gap appears in the middle of the modal element (I wrote this app, and this is how I'm certain this is a Firefox bug and not a Complice bug; it's literally impossible to produce that kind of knockout visual effect with CSS even if you're trying, as far as I'm aware). Expected results: The modal should scroll normally, eventually reaching the bottom with a Submit Outcomes button. (Sometimes it does this, but it's still weirdly pixellated.)

I will note that this is my first time using Bugzilla and I anticipate that "Web Apps" is not the right component for this bug, but I wasn't sure where else to put it!

I can't reproduce the screenshot exactly, but something strange is definitively going on: When scrolling down, the clip boundary of the popup moves upwards at twice the speed of the popup itself and so eventually the whole popup becomes invisible. I wonder whether there's any relation to bug 1445535, which is another instance of some page element moving at twice the scroll speed when it shouldn't...

Here is a reduced test case made from the URL that reproduces the problem.

The reduced testcase makes a debug build of Android crash with: 06-22 18:44:16.656 15376 15413 F MOZ_Assert: Assertion failure: IsAncestor(aOne, aTwo) || IsAncestor(aTwo, aOne), at /builds/worker/workspace/build/src/obj-firefox/dist/include/nsDisplayList.h:296

Bug 1427792 is on file to track that assertion failure. I'd suggest waiting for that to be fixed, and seeing if that resolves the rendering problem as well.