Auto merge of #24143 - gterzian:improve_spec_comp_bc_discarding, r=asajeffrey

Improve spec compliance of discarding BCs

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Came-up at https://github.com/servo/servo/pull/23637#issuecomment-528254988

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- [ ] These changes fix #___ (GitHub issue number if applicable)

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1 files changed, 28 insertions, 3 deletions

diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 440de991ceb..b4363f5ada3 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1973,7 +1973,15 @@ impl ScriptThread {
 
         let window = self.documents.borrow().find_window(pipeline_id);
         let window = match window {
-            Some(w) => w,
+            Some(w) => {
+                if w.Closed() {
+                    return warn!(
+                        "Received fire timer msg for a discarded browsing context whose pipeline is pending exit {}.",
+                        pipeline_id
+                    );
+                }
+                w
+            },
             None => {
                 return warn!(
                     "Received fire timer msg for a closed pipeline {}.",
@@ -2848,7 +2856,7 @@ impl ScriptThread {
             // to avoid running layout on detached iframes.
             let window = document.window();
             if discard_bc == DiscardBrowsingContext::Yes {
-                window.window_proxy().discard_browsing_context();
+                window.discard_browsing_context();
             }
             window.clear_js_runtime();
         }
@@ -3378,9 +3386,26 @@ impl ScriptThread {
     ///
     /// TODO: Actually perform DOM event dispatch.
     fn handle_event(&self, pipeline_id: PipelineId, event: CompositorEvent) {
+        // Do not handle events if the pipeline exited.
+        let window = match { self.documents.borrow().find_window(pipeline_id) } {
+            Some(win) => win,
+            None => {
+                return warn!(
+                    "Compositor event sent to a pipeline that already exited {}.",
+                    pipeline_id
+                )
+            },
+        };
+        // Do not handle events if the BC has been, or is being, discarded
+        if window.Closed() {
+            return warn!(
+                "Compositor event sent to a pipeline with a closed window {}.",
+                pipeline_id
+            );
+        }
+
         // Assuming all CompositionEvent are generated by user interactions.
         ScriptThread::set_user_interacting(true);
-
         match event {
             ResizeEvent(new_size, size_type) => {
                 self.handle_resize_event(pipeline_id, new_size, size_type);