LibWeb: Implement basic "scroll" events at the document level

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -320,6 +320,7 @@ void BrowsingContext::set_viewport_rect(Gfx::IntRect const& rect)
 
     if (m_viewport_scroll_offset != rect.location()) {
         m_viewport_scroll_offset = rect.location();
+        scroll_offset_did_change();
         did_change = true;
     }
 
@@ -785,4 +786,23 @@ DOM::Document* BrowsingContext::active_document()
     return m_active_document.cell();
 }
 
+void BrowsingContext::scroll_offset_did_change()
+{
+    // https://w3c.github.io/csswg-drafts/cssom-view-1/#scrolling-events
+    // Whenever a viewport gets scrolled (whether in response to user interaction or by an API), the user agent must run these steps:
+
+    // 1. Let doc be the viewport’s associated Document.
+    auto* doc = active_document();
+    VERIFY(doc);
+
+    // 2. If doc is already in doc’s pending scroll event targets, abort these steps.
+    for (auto& target : doc->pending_scroll_event_targets()) {
+        if (target.ptr() == doc)
+            return;
+    }
+
+    // 3. Append doc to doc’s pending scroll event targets.
+    doc->pending_scroll_event_targets().append(*doc);
+}
+
 }