The IPC to get cookies has to be synchronous. This causes a triangular deadlock between WebContent, the UI process, and RequestServer if we open a new tab from the UI while WebContent is requesting cookies.

• The WebContent process is spinning on send_sync_but_allow_failure<Messages::WebContentClient::DidRequestCookie>, waiting for the UI process to respond
• The UI process is spinning on send_sync_but_allow_failure<Messages::RequestServer::ConnectNewClient>, waiting for RequestServer to respond
• RequestServer is stuck in this loop, trying to write to the WebContent's socket file (when I attach to RS, we are always in the sched_yield call, so we're spinning on EAGAIN):
  

  ladybird/Userland/Services/RequestServer/ConnectionFromClient.cpp

  Lines 94 to 101 in 10acf3f

  	auto result = Core::System::write(request->writer_fd, { remaining_data, remaining_length });
  	if (result.is_error()) {
  	if (result.error().code() != EAGAIN) {
  	dbgln("on_data_received: write failed: {}", result.error());
  	VERIFY_NOT_REACHED();
  	}
  	sched_yield();
  	continue;

So it seems to me like there's 2 approaches:

1. Can we make the IPC from UI to RS async? @ADKaster
2. Instead of spinning on EAGAIN, we can e.g. do a deferred_invoke? @awesomekling

I'm most reliably able to repro this by navigating to youtube and opening a new tab after a couple seconds.