unknown completed load PageSource("https://arstechnica.com/")

[jdm]

While trying to reproduce #18872 I encountered this panic. I scrolled around the page a bunch after it appeared to finish loading, and then this appeared.

unknown completed load PageSource("https://arstechnica.com/") (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(NonZero(NonZero(7))) }, at /Users/jdm/src/master-servo/components/script/document_loader.rs:136)
stack backtrace:
   0:        0x1013ee0ee - backtrace::backtrace::trace::h255aedfedc5a51a4
   1:        0x1013ee12c - backtrace::capture::Backtrace::new::haedf38cc9f3a8a26
   2:        0x10002d4c8 - servo::main::_$u7b$$u7b$closure$u7d$$u7d$::hea4f1eedcb7bdb2d
   3:        0x1027e3621 - std::panicking::rust_panic_with_hook::he6f3840a88b909da
   4:        0x1027e3444 - std::panicking::begin_panic::h0084a9db51f07b23
   5:        0x1027e3392 - std::panicking::begin_panic_fmt::he894f42f5c15aa71
   6:        0x10079934b - script::document_loader::DocumentLoader::finish_load::hce58c576a8bc64ee
   7:        0x100842089 - script::dom::document::Document::finish_load::h2317dff16fa068fd
   8:        0x100975d98 - script::dom::servoparser::ServoParser::do_parse_sync::hc78e3dc07c8b7055
   9:        0x10097572a - script::dom::servoparser::ServoParser::parse_sync::h94dc4aea9b42d84e
  10:        0x100973a16 - script::dom::servoparser::ServoParser::resume_with_pending_parsing_blocking_script::hf180c3ffc079e7de
  11:        0x100842900 - script::dom::document::Document::process_pending_parsing_blocking_script::hc89a24232ab5d39c
  12:        0x10084261e - script::dom::document::Document::pending_parsing_blocking_script_loaded::hf05bbdcfb21bf67e
  13:        0x1008f38a9 - _$LT$script..dom..htmlscriptelement..ScriptContext$u20$as$u20$net_traits..FetchResponseListener$GT$::process_response_eof::heec6e4eef88ed8a2
  14:        0x100a2f00f - _$LT$script..network_listener..ListenerTask$LT$A$C$$u20$Listener$GT$$u20$as$u20$script..task..TaskOnce$GT$::run_once::h19a560e9d95e5e7c
  15:        0x10079104e - _$LT$T$u20$as$u20$script..task..TaskBox$GT$::run_box::ha48805f2993201e5
  16:        0x100a50270 - script::script_thread::ScriptThread::handle_msg_from_script::h9434b0b419dec3bf
  17:        0x100a495a2 - script::script_thread::ScriptThread::handle_msgs::_$u7b$$u7b$closure$u7d$$u7d$::h5615b992c2087765
  18:        0x100a45114 - script::script_thread::ScriptThread::handle_msgs::hc55a857665dc6624
  19:        0x1002f39d7 - std::sys_common::backtrace::__rust_begin_short_backtrace::h779d15d14ac1b071
  20:        0x1004e389d - std::panicking::try::do_call::hf3f5b1985d320558
  21:        0x1027f03cc - __rust_maybe_catch_panic
  22:        0x10061fbe5 - _$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$::call_box::h088a2a70e4535f54
  23:        0x1027e23fb - std::sys::imp::thread::Thread::new::thread_start::hf09d45f1d3cac3ae
  24:     0x7fffd9f7f93a - _pthread_body
  25:     0x7fffd9f7f886 - _pthread_start