servo: Merge #20181 - Fire the pageshow event at the end of the page load (from fabricedesre:end-load-pageshow); r=jdm

<!-- Please describe your changes on the following line: -->
This implements step 8 of https://html.spec.whatwg.org/multipage/parsing.html#the-end

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because I could not find a wpt test for that, which is strange. I likely missed it :(

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Source-Repo: https://github.com/servo/servo
Source-Revision: 94a6c2c429eb0415a277ff49fa19ae0eaefb0be9

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 78f055ebca3bf3519342874c13f60344e96f63fc

servo/components/atoms/static_atoms.txt

@@ -47,6 +47,7 @@ none
 number
 onchange
 open
+pageshow
 password
 pause
 play

servo/components/script/dom/document.rs

@@ -364,6 +364,8 @@ pub struct Document {
     canceller: FetchCanceller,
     /// https://html.spec.whatwg.org/multipage/#throw-on-dynamic-markup-insertion-counter
     throw_on_dynamic_markup_insertion_counter: Cell<u64>,
+    /// https://html.spec.whatwg.org/multipage/#page-showing
+    page_showing: Cell<bool>,
 }
 
 #[derive(JSTraceable, MallocSizeOf)]
@@ -1634,7 +1636,37 @@ impl Document {
         ).unwrap();
 
         // Step 8.
-        // TODO: pageshow event.
+        let document = Trusted::new(self);
+        if document.root().browsing_context().is_some() {
+            self.window.dom_manipulation_task_source().queue(
+                task!(fire_pageshow_event: move || {
+                    let document = document.root();
+                    let window = document.window();
+                    if document.page_showing.get() || !window.is_alive() {
+                        return;
+                    }
+
+                    document.page_showing.set(true);
+
+                    let event = PageTransitionEvent::new(
+                        window,
+                        atom!("pageshow"),
+                        false, // bubbles
+                        false, // cancelable
+                        false, // persisted
+                    );
+                    let event = event.upcast::<Event>();
+                    event.set_trusted(true);
+
+                    // FIXME(nox): Why are errors silenced here?
+                    let _ = window.upcast::<EventTarget>().dispatch_event_with_target(
+                        document.upcast(),
+                        &event,
+                    );
+                }),
+                self.window.upcast(),
+            ).unwrap();
+        }
 
         // Step 9.
         // TODO: pending application cache download process tasks.
@@ -2225,6 +2257,7 @@ impl Document {
             tti_window: DomRefCell::new(InteractiveWindow::new()),
             canceller: canceller,
             throw_on_dynamic_markup_insertion_counter: Cell::new(0),
+            page_showing: Cell::new(false),
         }
     }