Auto merge of #14344 - asajeffrey:script-thread-new-layout-use-incomplete-loads, r=jdm

Script thread creating layout thread should use the incomplete loads

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When a script thread creates a new layout thread, it does so by sending a message to an existing layout thread asking it to spawn. At the moment, we're only looking at the completed loads for that layout thread, so we can get a panic if two loads happen in quick succession. The temporary fix is to look for the layout thread in the incomplete loads too.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14333.
- [X] These changes do not require tests because it fixes a panic.

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components/script/script_thread.rs

@@ -1201,10 +1201,14 @@ impl ScriptThread {
         });
 
         // Pick a layout thread, any layout thread
-        match self.documents.borrow().iter().next() {
+        let current_layout_chan = self.documents.borrow().iter().next()
+            .map(|(_, document)| document.window().layout_chan().clone())
+            .or_else(|| self.incomplete_loads.borrow().first().map(|load| load.layout_chan.clone()));
+
+        match current_layout_chan {
             None => panic!("Layout attached to empty script thread."),
             // Tell the layout thread factory to actually spawn the thread.
-            Some((_, document)) => document.window().layout_chan().send(msg).unwrap(),
+            Some(layout_chan) => layout_chan.send(msg).unwrap(),
         };
 
         // Kick off the fetch for the new resource.