Webrender panics when scrolling a page back and forth

[jonathandturner]

If you open the latest servo (I tried on both macOS and Windows), open a site like reddit from the showcase. Next, scroll up and down on the page, stopping from time to time. After about 10 seconds for me, webrender panics, giving this backtrace:

called `Option::unwrap()` on a `None` value (thread RenderBackend, at src/libcore/option.rs:323)
stack backtrace:
   0:        0x10690076e - backtrace::backtrace::trace::hef11bbe02813de13
   1:        0x1069008d9 - backtrace::capture::Backtrace::new::hb5a725a088a2a2fc
   2:        0x10611aa1b - servo::main::_$u7b$$u7b$closure$u7d$$u7d$::h3467d0a38e82c110
   3:        0x1083a2d89 - std::panicking::rust_panic_with_hook::hdbc3bba6a9dc0bb9
   4:        0x1083a2be4 - std::panicking::begin_panic::ha249c774b9bf96e1
   5:        0x1083a2b52 - std::panicking::begin_panic_fmt::ha197daaf02a649ab
   6:        0x1083a2ab7 - rust_begin_unwind
   7:        0x1083caf80 - core::panicking::panic_fmt::h7414cb5ce71ea361
   8:        0x1083cae84 - core::panicking::panic::hcbe9ad6a1a4f4afd
   9:        0x1066c3b6c - webrender::frame_builder::FrameBuilder::build_render_task::hb9aba9e851f2e67b
  10:        0x1066c3e95 - webrender::frame_builder::FrameBuilder::build::h9dfc3dbbe562f073
  11:        0x1066bd223 - webrender::frame::Frame::build_frame::h3cb018dc18d4f91c
  12:        0x1066bd12f - webrender::frame::Frame::build::h3eb439af07cdd7a2
  13:        0x1066d9a88 - webrender::render_backend::RenderBackend::render::h7d725c99525b370d
  14:        0x1066d381d - webrender::profiler::TimeProfileCounter::profile::h80648d428f30f92a
  15:        0x1066d6012 - webrender::render_backend::RenderBackend::run::h586004cebbd294f6
  16:        0x10667f9c8 - _$LT$std..panic..AssertUnwindSafe$LT$F$GT$$u20$as$u20$core..ops..FnOnce$LT$$LP$$RP$$GT$$GT$::call_once::h12e6760be9e1aa79
  17:        0x106681aad - std::panicking::try::do_call::hfd5d9d1d1c64810e
  18:        0x1083a3cba - __rust_maybe_catch_panic
  19:        0x106681a0b - std::panicking::try::h642e9b8674cf05c0
  20:        0x10667e46f - std::panic::catch_unwind::hd7f81fb4166a708b
  21:        0x1066ab6ff - _$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$::call_box::hd4f6c63a2c1d06fd
  22:        0x10839f914 - std::sys::imp::thread::Thread::new::thread_start::h4008e1859fbd98b8
  23:     0x7fffe299baaa - _pthread_body
  24:     0x7fffe299b9f6 - _pthread_start
ERROR:servo: called `Option::unwrap()` on a `None` value
called `Result::unwrap()` on an `Err` value: IoError(Error { repr: Custom(Custom { kind: Other, error: StringError("Unknown Mach error: 46") }) }) (thread main, at src/libcore/result.rs:860)
called `Result::unwrap()` on an `Err` value: IoError(Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) }) (thread LayoutThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(2) }, at src/libcore/result.rs:860)
stack backtrace:
   0:        0x10690076e - backtrace::backtrace::trace::hef11bbe02813de13
   1:        0x1069008d9 - backtrace::capture::Backtrace::new::hb5a725a088a2a2fc
   2:        0x10611aa1b - servo::main::_$u7b$$u7b$closure$u7d$$u7d$::h3467d0a38e82c110
   3:        0x1083a2d89 - std::panicking::rust_panic_with_hook::hdbc3bba6a9dc0bb9
   4:        0x1083a2be4 - std::panicking::begin_panic::ha249c774b9bf96e1
   5:        0x1083a2b52 - std::panicking::begin_panic_fmt::ha197daaf02a649ab
   6:        0x1083a2ab7 - rust_begin_unwind
   7:        0x1083caf80 - core::panicking::panic_fmt::h7414cb5ce71ea361
   8:        0x1082f7f64 - core::result::unwrap_failed::h06ffc6135155777a
   9:        0x1083035f1 - webrender_traits::api::RenderApi::get_scroll_layer_state::h1ca0c294a0934069
  10:        0x1060f88c9 - _$LT$compositing..compositor..IOCompositor$LT$Window$GT$$GT$::send_viewport_rects::h753085534eee3a63
  11:        0x1060fd134 - _$LT$compositing..compositor..IOCompositor$LT$Window$GT$$GT$::process_pending_scroll_events::hc7819da65b4fee67
  12:        0x1060f7b23 - _$LT$compositing..compositor..IOCompositor$LT$Window$GT$$GT$::handle_events::hee4f3384109ec527
  13:        0x10611ad57 - _$LT$servo..Browser$LT$Window$GT$$GT$::handle_events::h0557f4d128c0d8df
  14:        0x10611a3ca - servo::main::hccd3cbec30c8eceb
  15:        0x1083a2a15 - std::panicking::try::do_call::h24a2756282b9a31c
  16:        0x1083a3cba - __rust_maybe_catch_panic
  17:        0x1083a3156 - std::rt::lang_start::hd19f94db0c6a490e
ERROR:servo: called `Result::unwrap()` on an `Err` value: IoError(Error { repr: Custom(Custom { kind: Other, error: StringError("Unknown Mach error: 46") }) })
assertion failed: mach_sys::mach_port_mod_refs(mach_task_self(), self.port,
                             MACH_PORT_RIGHT_SEND, 1) == KERN_SUCCESS (thread Constellation, at /Users/jturner/Source/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/ipc-channel-0.7.0/src/platform/macos/mod.rs:336)
Servo exited with return value 101

[ferjm]

I am seeing this with Google Docs as well #13942.

With Reddit, it takes some time to crash, as you mentioned. With Docs it happens with the first scroll down.