LibWeb: Cache state of the contenteditable attribute on HTMLElement

Instead of recomputing the state whenever someone asks for it, we now
cache it when the attribute is added/changed/removed.

Before this change, HTMLElement::is_editable() was 6.5% of CPU time
when furiously resizing Hacker News. After, it's less than 0.5%. :^)

2 files changed, 25 insertions, 16 deletions

diff --git a/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp
index 817b364655..6dc2953e0b 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp
@@ -77,22 +77,9 @@ void HTMLElement::set_dir(DeprecatedString const& dir)
     MUST(set_attribute(HTML::AttributeNames::dir, dir));
 }
 
-HTMLElement::ContentEditableState HTMLElement::content_editable_state() const
-{
-    auto contenteditable = attribute(HTML::AttributeNames::contenteditable);
-    // "true", an empty string or a missing value map to the "true" state.
-    if ((!contenteditable.is_null() && contenteditable.is_empty()) || contenteditable.equals_ignoring_ascii_case("true"sv))
-        return ContentEditableState::True;
-    // "false" maps to the "false" state.
-    if (contenteditable.equals_ignoring_ascii_case("false"sv))
-        return ContentEditableState::False;
-    // Having no such attribute or an invalid value maps to the "inherit" state.
-    return ContentEditableState::Inherit;
-}
-
 bool HTMLElement::is_editable() const
 {
-    switch (content_editable_state()) {
+    switch (m_content_editable_state) {
     case ContentEditableState::True:
         return true;
     case ContentEditableState::False:
@@ -106,7 +93,7 @@ bool HTMLElement::is_editable() const
 
 DeprecatedString HTMLElement::content_editable() const
 {
-    switch (content_editable_state()) {
+    switch (m_content_editable_state) {
     case ContentEditableState::True:
         return "true";
     case ContentEditableState::False:
@@ -242,6 +229,19 @@ void HTMLElement::parse_attribute(DeprecatedFlyString const& name, DeprecatedStr
 {
     Element::parse_attribute(name, value);
 
+    if (name == HTML::AttributeNames::contenteditable) {
+        if ((!value.is_null() && value.is_empty()) || value.equals_ignoring_ascii_case("true"sv)) {
+            // "true", an empty string or a missing value map to the "true" state.
+            m_content_editable_state = ContentEditableState::True;
+        } else if (value.equals_ignoring_ascii_case("false"sv)) {
+            // "false" maps to the "false" state.
+            m_content_editable_state = ContentEditableState::False;
+        } else {
+            // Having no such attribute or an invalid value maps to the "inherit" state.
+            m_content_editable_state = ContentEditableState::Inherit;
+        }
+    }
+
     // 1. If namespace is not null, or localName is not the name of an event handler content attribute on element, then return.
     // FIXME: Add the namespace part once we support attribute namespaces.
 #undef __ENUMERATE
@@ -253,6 +253,14 @@ void HTMLElement::parse_attribute(DeprecatedFlyString const& name, DeprecatedStr
 #undef __ENUMERATE
 }
 
+void HTMLElement::did_remove_attribute(DeprecatedFlyString const& name)
+{
+    Base::did_remove_attribute(name);
+    if (name == HTML::AttributeNames::contenteditable) {
+        m_content_editable_state = ContentEditableState::Inherit;
+    }
+}
+
 // https://html.spec.whatwg.org/multipage/interaction.html#dom-focus
 void HTMLElement::focus()
 {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLElement.h b/Userland/Libraries/LibWeb/HTML/HTMLElement.h
index 4bfc1c6a96..86bda36d39 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLElement.h
@@ -67,6 +67,7 @@ protected:
     virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
 
     virtual void parse_attribute(DeprecatedFlyString const& name, DeprecatedString const& value) override;
+    virtual void did_remove_attribute(DeprecatedFlyString const& name) override;
 
     virtual void visit_edges(Cell::Visitor&) override;
 
@@ -81,7 +82,7 @@ private:
         False,
         Inherit,
     };
-    ContentEditableState content_editable_state() const;
+    ContentEditableState m_content_editable_state { ContentEditableState::Inherit };
 
     JS::GCPtr<DOMStringMap> m_dataset;