Move checks for document completion to the end of the event loop.

This better reflects the text of the specification - rather than
queuing a task to dispatch the load evnet as soon as the document
loader is unblocked, we want to "spin the event loop until there
is nothing that delays the load event in the Document." Spinning
the event loop is a concept that requires running tasks
completely, hence we check the condition before returning to the
start of the event loop.

components/script/dom/document.rs

@@ -107,7 +107,7 @@ use net_traits::response::HttpsState;
 use num_traits::ToPrimitive;
 use script_layout_interface::message::{Msg, ReflowQueryType};
 use script_runtime::{CommonScriptMsg, ScriptThreadEventCategory};
-use script_thread::{MainThreadScriptMsg, Runnable};
+use script_thread::{MainThreadScriptMsg, Runnable, ScriptThread};
 use script_traits::{AnimationState, CompositorEvent, DocumentActivity};
 use script_traits::{MouseButton, MouseEventType, MozBrowserEvent};
 use script_traits::{ScriptMsg as ConstellationMsg, TouchpadPressurePhase};
@@ -1599,17 +1599,26 @@ impl Document {
         // Step 5 can be found in asap_script_loaded and
         // asap_in_order_script_loaded.
 
+        let loader = self.loader.borrow();
+        if loader.is_blocked() || loader.events_inhibited() {
+            // Step 6.
+            return;
+        }
+
+        ScriptThread::mark_document_with_no_blocked_loads(self);
+    }
+
+    // https://html.spec.whatwg.org/multipage/#the-end
+    pub fn maybe_queue_document_completion(&self) {
         if self.loader.borrow().is_blocked() {
             // Step 6.
             return;
         }
 
-        // The rest will ever run only once per document.
-        if self.loader.borrow().events_inhibited() {
-            return;
-        }
+        assert!(!self.loader.borrow().events_inhibited());
         self.loader.borrow_mut().inhibit_events();
 
+        // The rest will ever run only once per document.
         // Step 7.
         debug!("Document loads are complete.");
         let handler = box DocumentProgressHandler::new(Trusted::new(self));