LibWeb: Add default user agent style sheet for Fullscreen API

Make sure matches_pseudo_class also returns for the pseudo-class
:fullscreen.

Also added a new invalidation reason, for when we want to update due to
invalidation (after we "set" the pseudo-class :fullscreen on our
element(s)).

For now Element::is_fullscreen_element just returns false. In future
commits, `Element` will receive a fullscreen flag that can be set.

Libraries/LibWeb/CSS/Default.css

@@ -959,6 +959,31 @@ button, meter, progress, select {
   animation-fill-mode: inherit;
   animation-delay: inherit;
 }
+/* Fullscreen API defaults https://fullscreen.spec.whatwg.org/#user-agent-level-style-sheet-defaults */
+*|*:not(:root):fullscreen {
+  position:fixed !important;
+  inset:0 !important;
+  margin:0 !important;
+  box-sizing:border-box !important;
+  min-width:0 !important;
+  max-width:none !important;
+  min-height:0 !important;
+  max-height:none !important;
+  width:100% !important;
+  height:100% !important;
+  transform:none !important;
+    /* intentionally not !important */
+  object-fit:contain;
+}
+
+iframe:fullscreen {
+  border:none !important;
+  padding:0 !important;
+}
+
+*|*:not(:root):fullscreen::backdrop {
+  background:black;
+}
 
 /* Default cross-fade transition */
 @keyframes -ua-view-transition-fade-out {

Libraries/LibWeb/CSS/PseudoClasses.json

@@ -44,6 +44,9 @@
   "focus-within": {
     "argument": ""
   },
+  "fullscreen": {
+    "argument": ""
+  },
   "has": {
     "argument": "<relative-selector-list>"
   },

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -8,6 +8,7 @@
 #include <LibWeb/CSS/ComputedProperties.h>
 #include <LibWeb/CSS/Keyword.h>
 #include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/CSS/PseudoClass.h>
 #include <LibWeb/CSS/SelectorEngine.h>
 #include <LibWeb/DOM/Attr.h>
 #include <LibWeb/DOM/Document.h>
@@ -461,6 +462,9 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         auto* focused_element = element.document().focused_element();
         return focused_element && element.is_inclusive_ancestor_of(*focused_element);
     }
+    case CSS::PseudoClass::Fullscreen: {
+        return element.is_fullscreen_element();
+    }
     case CSS::PseudoClass::FirstChild:
         if (context.collect_per_element_selector_involvement_metadata) {
             const_cast<DOM::Element&>(element).set_affected_by_first_or_last_child_pseudo_class(true);

Libraries/LibWeb/DOM/Element.cpp

@@ -3235,6 +3235,11 @@ size_t Element::ordinal_value() const
     return numbering;
 }
 
+bool Element::is_fullscreen_element() const
+{
+    return false;
+}
+
 bool Element::id_reference_exists(String const& id_reference) const
 {
     return document().get_element_by_id(id_reference);

Libraries/LibWeb/DOM/Element.h

@@ -471,6 +471,8 @@ public:
     void release_pointer_capture(WebIDL::Long pointer_id);
     bool has_pointer_capture(WebIDL::Long pointer_id);
 
+    bool is_fullscreen_element() const;
+
 protected:
     Element(Document&, DOM::QualifiedName);
     virtual void initialize(JS::Realm&) override;

Libraries/LibWeb/DOM/Node.h

@@ -59,6 +59,8 @@ enum class ShouldComputeRole {
     X(EditingInsertion)                             \
     X(ElementAttributeChange)                       \
     X(ElementSetShadowRoot)                         \
+    X(FocusedElementChange)                         \
+    X(Fullscreen)                                   \
     X(HTMLHyperlinkElementHrefChange)               \
     X(HTMLIFrameElementGeometryChange)              \
     X(HTMLInputElementSetChecked)                   \