'LayoutTask' failed at 'no layout root' in Document-createElement-namespace.html

[mbrubeck]

This crash showed up in web-platform-tests/dom/nodes/Document-createElement-namespace.html starting with PR #3721:

task 'LayoutTask' failed at 'no layout root', /Users/larsberg/rust/src/libcore/option.rs:314
stack backtrace:
   1:        0x105d52fa9 - rt::backtrace::imp::write::he90f55e975dac19a1Tq
   2:        0x105d56495 - failure::on_fail::h1aa28f9ec3f7fda6Far
   3:        0x105d6b625 - unwind::begin_unwind_inner::h97f5bca93429abcaPQd
   4:        0x105d6b2e3 - unwind::begin_unwind_fmt::h904d273690034602hOd
   5:        0x105d6b052 - rust_begin_unwind
   6:        0x105d905ec - failure::begin_unwind::h075fd6b86e2794c3opk
   7:        0x105d90160 - atomic::AtomicInt::new::hb69abbaebf8ff771Gwj
   8:        0x104d25820 - option::Option<T>::expect::h10717376083701670376
   9:        0x104d25724 - layout_task::LayoutTask::get_layout_root::h075837dae328b28c4ij
  10:        0x104d270ec - layout_task::LayoutTask::handle_reflow::closure.29200
  11:        0x104d26d58 - time::profile::h9488975495268010682
  12:        0x104d1e2db - layout_task::LayoutTask::handle_reflow::hbf9a506294ef7ed7zmj
  13:        0x104d1d7e7 - layout_task::LayoutTask::handle_script_request::closure.28988
  14:        0x104d1a863 - time::profile::h6610779894107480767
  15:        0x104d11c85 - layout_task::LayoutTask::handle_script_request::h987ca309d061944eP6i
  16:        0x104cff740 - layout_task::LayoutTask::handle_request::h83aa9b44144733d0v3i
  17:        0x104cd7c52 - layout_task::LayoutTask::start::h68a8bf743dc794b6h2i
  18:        0x104cc5be5 - layout_task::LayoutTask.LayoutTaskFactory::create::closure.27473
  19:        0x105b85f02 - rtinstrument::instrument::h707780a2f08d86aaFLd
  20:        0x104cad031 - task::spawn_named_with_send_on_failure::closure.26980
  21:        0x104cc4246 - task::TaskBuilder<S>::try_future::closure.27411
  22:        0x1057734bd - task::bootstrap_green_task::closure.6832
  23:        0x105d6c27c - rust_try_inner
  24:        0x105d6c266 - rust_try
  25:        0x105d68ea7 - unwind::try::hef59009c1de381e8xFd
  26:        0x105d68d5c - task::Task::run::h6af0212431b92fd5MVc
  27:        0x105773380 - task::bootstrap_green_task::hcb2cc30f27b6339465b

http://build.servo.org/builders/mac1/builds/30/steps/shell_1/logs/stdio

[dralley]

Can't reproduce, although it does print

[2020-03-25T13:41:19Z ERROR script::dom::bindings::error] Error at file:///home/dalley/Devel/servo-dev/servo/tests/wpt/web-platform-tests/dom/nodes/Document-createElement-namespace.html:34:1 test is not defined

[jdm]

This test gets run successfully as part of our CI.