**Steps to reproduce** 1. Go to `data:text/html,<meta charset=utf8><style>h1 {animation: 1.1s linear appears;}@keyframes appears {0% {opacity: 0;}100% {opacity: 1;}}</style><h1>Hello</h1>` **Expected results** The "Hello" text slowly appears. At the end of the animation, the text stays visible **Actual results** Sometimes, there's a flash at the end of the animation (see attached screencast, at 2-3s). You can reload the tab a few times, I usually reproduce this in 3 tries. This only seems to happen with webRender enabled ---

What platform do you see this on?

OSX

I tried the STR in comment 0 with latest nightly on osx, win10 and linux, but could not reproduce the problem.

It might be similar to Bug 1497852.

Getting a WR capture here would probably help. Might not be too hard to modify the code to just grab a capture for every frame in an OMTA and then just pick out the bad frame.

Nicolas, can you reproduce this problem on Windows?

I could reproduce the problem by adding latency in LowPrioritySceneBuilder::process_transaction() from it. The problem seemed to be caused by async animation.

With attachment 9034650 [details] [diff] [review] , I reproduce the parobem with STR in comment 0. And when I disabled async animation by pref layers.offmainthreadcomposition.async-animations:false, I did not see the problem.

WebRenderBridgeParent::RemoveEpochDataPriorTo() seems to delete animation too early. WebRenderLayerManager::DiscardCompositorAnimations() was called just before calling WrBridge()->EndTransaction() in WebRenderLayerManager::EndTransactionWithoutLayer(). Then WebRenderBridgeParent::RecvDeleteCompositorAnimations() was called just before WebRenderBridgeParent::RecvSetDisplayList() call. The RecvSetDisplayList() update WrEpoch. Then the RecvDeleteCompositorAnimations() stored old epoch in mCompositorAnimationsToDelete. https://dxr.mozilla.org/mozilla-central/source/gfx/layers/wr/WebRenderBridgeParent.cpp#778

It seems better to call WebRenderLayerManager::DiscardCompositorAnimations() after calling WrBridge()->EndTransaction() in WebRenderLayerManager::EndTransactionWithoutLayer().

> Nicolas, can you reproduce this problem on Windows? I tried with the STR, reloading the page ~50 times and couldn't reproduce it.

Pushed by sikeda@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/ed51ef14289e Call DiscardCompositorAnimations() after calling WrBridge()->EndTransaction() r=kats