Intermittent crash in /_mozilla/css/submit_focus_a.html

[nox]

  ▶ CRASH [expected PASS] /_mozilla/css/submit_focus_a.html
  │ 
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ thread 'ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }' panicked at 'index out of bounds: the len is 0 but the index is 0', ../src/libcollections/vec.rs:1167
  │ stack backtrace:
  │    1:        0x10f8f2ac8 - std::sys::backtrace::tracing::imp::write::h9fb600083204ae7f
  │    2:        0x10f8f8e75 - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::hca543c34f11229ac
  │    3:        0x10f8f8a8e - std::panicking::default_hook::hc2c969e7453d080c
  │    4:        0x10f6d3a32 - util::panicking::initiate_panic_hook::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::ha2bd86c312dc8d7a
  │    5:        0x10f8e0aa2 - std::panicking::rust_panic_with_hook::hfe203e3083c2b544
  │    6:        0x10f8f9436 - std::panicking::begin_panic::h4889569716505182
  │    7:        0x10f8e2308 - std::panicking::begin_panic_fmt::h484cd47786497f03
  │    8:        0x10f8f908f - rust_begin_unwind
  │    9:        0x10f91ff10 - core::panicking::panic_fmt::h257ceb0aa351d801
  │   10:        0x10f920090 - core::panicking::panic_bounds_check::h3956fdcea61ff421
  │   11:        0x10e60eb3e - script::dom::browsingcontext::BrowsingContext::active_document::h226f24d08ec44427
  │   12:        0x10e61385a - script::dom::browsingcontext::BrowsingContext::active_window::h2ba128d21d9d19cf
  │   13:        0x10ec2777c - _<script..script_thread..ScriptMemoryFailsafe<'a> as core..ops..Drop>::drop::h4fd0a77cbabe1834
  │   14:        0x10ec2b9ee - std::panicking::try::call::hc9179c9e89d6d659
  │   15:        0x10f8fc11b - __rust_try
  │   16:        0x10f8fc0b5 - __rust_maybe_catch_panic
  │   17:        0x10ec2c1b4 - _<F as alloc..boxed..FnBox<A>>::call_box::hbef70c0af4cb429b
  │   18:        0x10f8f7dd8 - std::sys::thread::Thread::new::thread_start::h6f266e069bf4ec2b
  │   19:     0x7fff8e158059 - _pthread_body
  │   20:     0x7fff8e157fd6 - _pthread_start
  └ thread panicked while panicking. aborting.

[jdm]

Same panic seen in #11400.