The FlushSceneBuilds() does sync ipc with SceneBuilder thread. It is nice if we could reduce sync ipc.

How often/when does this happen?

(In reply to Jeff Muizelaar [:jrmuizel] from comment #1) > How often/when does this happen? FlushSceneBuilds() is called in the following situations. -[1] WebRenderBridgeParent::FlushRendering() + When widget invalidation is necessary. https://dxr.mozilla.org/mozilla-central/source/view/nsViewManager.cpp#351 -[2] WebRenderBridgeParent::ClearResources() + when Tab is closed. + When new window is cread by tab move. -[3] WebRenderBridgeParent::RecvGetSnapshot() + This does not happen in normal browsing use case. + This happen during testing. -[4] WebRenderBridgeParent::RecvSyncWithCompositor() + Triggered by PersistentBufferProviderShared when it needs many TextureClients. + This does not happen in normal use case. Frequency of [1] seems different between platforms. On linux and on windows, [1] was called during resizing windows. On mac, [1] was called more often(like Tab move within window).

With attachment 9004747 [details] [diff] [review], I checked the waiting duration. The duration seemed to depend on content page complexity. I saw 10ms-30ms during resizing window on https://www.yahoo.co.jp/. But during https://ci.chromium.org/p/chromium/builders case, the duration was 100ms - 200ms.

It is already partially done by Bug 1479912.