called mark_as_root() on an unhandled flow

[mateon1]

When running the following testcase (accidentally found while reducing #18544), a panic is hit:

<script> document.children[0].style["column-count"] = 1 </script>

called mark_as_root() on an unhandled flow (thread LayoutThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }, at /shared/dev/rust/servo/components/layout/flow.rs:374)
stack backtrace:
   0:     0x562ac8905184 - backtrace::backtrace::libunwind::trace
                        at /shared/dev/rust/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace<closure>
                        at /shared/dev/rust/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
   1:     0x562ac88ff9af - backtrace::capture::{{impl}}::new
                        at /shared/dev/rust/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/capture.rs:64
   2:     0x562ac1e63ab6 - servo::main::{{closure}}
                        at /shared/dev/rust/servo/ports/servo/main.rs:130
   3:     0x562ac8be5146 - std::panicking::rust_panic_with_hook
                        at /checkout/src/libstd/panicking.rs:578
   4:     0x562ac29418a5 - std::panicking::begin_panic<&str>
                        at /checkout/src/libstd/panicking.rs:538
   5:     0x562ac2885349 - layout::flow::Flow::mark_as_root<layout::multicol::MulticolFlow>
                        at /shared/dev/rust/servo/ports/servo/<panic macros>:3
   6:     0x562ac27b70a2 - layout_thread::{{impl}}::try_get_layout_root<layout_thread::dom_wrapper::ServoLayoutNode>
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:907
   7:     0x562ac27ba571 - layout_thread::{{impl}}::handle_reflow
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:1304
   8:     0x562ac2a4963d - layout_thread::{{impl}}::handle_request_helper::{{closure}}
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:699
   9:     0x562ac290c983 - profile_traits::time::profile<(),closure>
                        at /shared/dev/rust/servo/components/profile_traits/time.rs:124
  10:     0x562ac27b460e - layout_thread::{{impl}}::handle_request_helper
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:696
  11:     0x562ac27b33ab - layout_thread::{{impl}}::handle_request
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:643
  12:     0x562ac27b26c3 - layout_thread::{{impl}}::start
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:556
  13:     0x562ac2b10d29 - layout_thread::{{impl}}::create::{{closure}}::{{closure}}
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:316
  14:     0x562ac2a63fa4 - profile_traits::mem::{{impl}}::run_with_memory_reporting<closure,fn(profile_traits::mem::ReportsChan) -> script_layout_interface::message::Msg,script_layout_interface::message::Msg,std::sync::mpsc::Sender<script_layout_interface::message::Msg>>
                        at /shared/dev/rust/servo/components/profile_traits/mem.rs:63
  15:     0x562ac28ccfb7 - layout_thread::{{impl}}::create::{{closure}}
                        at /shared/dev/rust/servo/components/layout_thread/lib.rs:315
  16:     0x562ac281023a - std::sys_common::backtrace::__rust_begin_short_backtrace<closure,()>
                        at /checkout/src/libstd/sys_common/backtrace.rs:136
  17:     0x562ac2811fb3 - std::thread::{{impl}}::spawn::{{closure}}::{{closure}}<closure,()>
                        at /checkout/src/libstd/thread/mod.rs:394
  18:     0x562ac27c0dda - std::panic::{{impl}}::call_once<(),closure>
                        at /checkout/src/libstd/panic.rs:296
  19:     0x562ac2942c62 - std::panicking::try::do_call<std::panic::AssertUnwindSafe<closure>,()>
                        at /checkout/src/libstd/panicking.rs:480
  20:     0x562ac8bec2dc - panic_unwind::__rust_maybe_catch_panic
                        at /checkout/src/libpanic_unwind/lib.rs:99
  21:     0x562ac29419ec - std::panicking::try<(),std::panic::AssertUnwindSafe<closure>>
                        at /checkout/src/libstd/panicking.rs:459
  22:     0x562ac2810fb5 - std::panic::catch_unwind<std::panic::AssertUnwindSafe<closure>,()>
                        at /checkout/src/libstd/panic.rs:361
  23:     0x562ac2811ddb - std::thread::{{impl}}::spawn::{{closure}}<closure,()>
                        at /checkout/src/libstd/thread/mod.rs:393
  24:     0x562ac2859933 - alloc::boxed::{{impl}}::call_box<(),closure>
                        at /checkout/src/liballoc/boxed.rs:682
  25:     0x562ac8be3dcb - alloc::boxed::{{impl}}::call_once<(),()>
                        at /checkout/src/liballoc/boxed.rs:692
                         - std::sys_common::thread::start_thread
                        at /checkout/src/libstd/sys_common/thread.rs:21
                         - std::sys::imp::thread::{{impl}}::new::thread_start
                        at /checkout/src/libstd/sys/unix/thread.rs:84
  26:     0x7f267df7f493 - start_thread
  27:     0x7f267daacabe - __clone
  28:                0x0 - <unknown>
ERROR:servo: called mark_as_root() on an unhandled flow

[mateon1]

Additional, simpler testcase. Found with domato.

<style>
html { display: list-item; }
</style>

[dralley]

This can still be reproduced.