LibWeb: Implement plumbing for view transitions

This implements large parts of the CSS view-transitions-1 spec.

Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -458,7 +458,10 @@ void EventLoop::update_the_rendering()
 
     // FIXME: 17. For each doc of docs, if the focused area of doc is not a focusable area, then run the focusing steps for doc's viewport, and set doc's relevant global object's navigation API's focus changed during ongoing navigation to false.
 
-    // FIXME: 18. For each doc of docs, perform pending transition operations for doc. [CSSVIEWTRANSITIONS]
+    // 18. For each doc of docs, perform pending transition operations for doc. [CSSVIEWTRANSITIONS]
+    for (auto& document : docs) {
+        document->perform_pending_transition_operations();
+    }
 
     // 19. For each doc of docs, run the update intersection observations steps for doc, passing in the relative high resolution time given now and doc's relevant global object as the timestamp. [INTERSECTIONOBSERVER]
     for (auto& document : docs) {

Libraries/LibWeb/HTML/Navigable.cpp

@@ -2587,6 +2587,22 @@ void Navigable::paste(String const& text)
     m_event_handler.handle_paste(text);
 }
 
+// https://drafts.csswg.org/css-view-transitions-1/#snapshot-containing-block
+CSSPixelRect Navigable::snapshot_containing_block()
+{
+    // The snapshot containing block is a rectangle that covers all areas of the window that could potentially display
+    // page content (and is therefore consistent regardless of root scrollbars or interactive widgets).
+
+    // Within a child navigable, the snapshot containing block is the union of the navigable’s viewport with any scrollbar gutters.
+    // FIXME: Actually get the correct rectangle here.
+    return viewport_rect();
+}
+// https://drafts.csswg.org/css-view-transitions-1/#snapshot-containing-block-size
+CSSPixelSize Navigable::snapshot_containing_block_size()
+{
+    return this->snapshot_containing_block().size();
+}
+
 void Navigable::register_navigation_observer(Badge<NavigationObserver>, NavigationObserver& navigation_observer)
 {
     auto result = m_navigation_observers.set(navigation_observer);

Libraries/LibWeb/HTML/Navigable.h

@@ -9,6 +9,7 @@
 
 #include <AK/HashTable.h>
 #include <AK/String.h>
+#include <AK/Tuple.h>
 #include <LibJS/Heap/Cell.h>
 #include <LibWeb/Bindings/NavigationPrototype.h>
 #include <LibWeb/DOM/DocumentLoadEventDelayer.h>
@@ -200,6 +201,11 @@ public:
     Web::EventHandler& event_handler() { return m_event_handler; }
     Web::EventHandler const& event_handler() const { return m_event_handler; }
 
+    // https://drafts.csswg.org/css-view-transitions-1/#snapshot-containing-block
+    CSSPixelRect snapshot_containing_block();
+    // https://drafts.csswg.org/css-view-transitions-1/#snapshot-containing-block-size
+    CSSPixelSize snapshot_containing_block_size();
+
     bool has_session_history_entry_and_ready_for_navigation() const { return m_has_session_history_entry_and_ready_for_navigation; }
     void set_has_session_history_entry_and_ready_for_navigation();