https://treeherder.mozilla.org/logviewer.html#?job_id=81268577&repo=graphics&lineNumber=6290 thread 'RenderBackend' panicked at 'no entry found for key', /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/option.rs:715 stack backtrace: 1: 0x7f7cc2f9119a - std::sys::imp::backtrace::tracing::imp::write::h3188f035833a2635 2: 0x7f7cc2f905ef - std::panicking::default_hook::{{closure}}::h6385b6959a2dd25b 3: 0x7f7cc2f8fc3c - std::panicking::rust_panic_with_hook::hf00b8130f73095ec 4: 0x7f7cc2f8f7bf - std::panicking::begin_panic::h6227f62cb2cdaeb4 5: 0x7f7cc2f8f769 - std::panicking::begin_panic_fmt::h173eadd80ae64bec 6: 0x7f7cc2f9a516 - core::panicking::panic_fmt::h3b2d1e30090844ff 7: 0x7f7cc2f9f6d7 - core::option::expect_failed::h5acccc61e40ad23f 8: 0x7f7cc2f34077 - webrender::frame::Frame::flatten_items::h4502b92dd7225db8 9: 0x7f7cc2f325be - webrender::frame::Frame::flatten_items::h4502b92dd7225db8 10: 0x7f7cc2f2d4f8 - webrender::frame::Frame::flatten_stacking_context::h5dee7bbc613e4479 11: 0x7f7cc2f32070 - webrender::frame::Frame::flatten_items::h4502b92dd7225db8 12: 0x7f7cc2f325be - webrender::frame::Frame::flatten_items::h4502b92dd7225db8 13: 0x7f7cc2f2d4f8 - webrender::frame::Frame::flatten_stacking_context::h5dee7bbc613e4479 14: 0x7f7cc2f32070 - webrender::frame::Frame::flatten_items::h4502b92dd7225db8 15: 0x7f7cc2f325be - webrender::frame::Frame::flatten_items::h4502b92dd7225db8 16: 0x7f7cc2f2d4f8 - webrender::frame::Frame::flatten_stacking_context::h5dee7bbc613e4479 17: 0x7f7cc2f32070 - webrender::frame::Frame::flatten_items::h4502b92dd7225db8 18: 0x7f7cc2f2d4f8 - webrender::frame::Frame::flatten_stacking_context::h5dee7bbc613e4479 19: 0x7f7cc2f28c8f - webrender::render_backend::RenderBackend::build_scene::h1a43e07d7cd5b4c5 20: 0x7f7cc2f1324f - webrender::render_backend::RenderBackend::run::h12c48ce9085dd5bd 21: 0x7f7cc2f0b34d - <F as alloc::boxed::FnBox<A>>::call_box::h7edba8096fdca8a6 22: 0x7f7cc2f97d03 - std::sys::imp::thread::Thread::new::thread_start::he018521f53b24939 23: 0x7f7ccff286b9 - start_thread 24: 0x7f7ccefb182c - clone 25: 0x0 - <unknown>

I guess we haven't seen this in a while.