LibWeb: Implement popover property and attribute

The popover property now reflects the attribute.

It cannot currently use the Reflect IDL concept because the empty string
value is special cased.

(cherry picked from commit a9669639cee27e81640fe7a37d9d1f319db6b666;
amended to undo unintentional -- and conflicting -- change to
Tests/LibWeb/rebaseline-libweb-test)

Userland/Libraries/LibWeb/HTML/AttributeNames.h

@@ -214,6 +214,7 @@ namespace AttributeNames {
     __ENUMERATE_HTML_ATTRIBUTE(ping)                       \
     __ENUMERATE_HTML_ATTRIBUTE(placeholder)                \
     __ENUMERATE_HTML_ATTRIBUTE(playsinline)                \
+    __ENUMERATE_HTML_ATTRIBUTE(popover)                    \
     __ENUMERATE_HTML_ATTRIBUTE(poster)                     \
     __ENUMERATE_HTML_ATTRIBUTE(preload)                    \
     __ENUMERATE_HTML_ATTRIBUTE(readonly)                   \

Userland/Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -681,6 +681,34 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<ElementInternals>> HTMLElement::attach_inte
     return { internals };
 }
 
+// https://html.spec.whatwg.org/multipage/popover.html#dom-popover
+Optional<String> HTMLElement::popover() const
+{
+    // FIXME: This should probably be `Reflect` in the IDL.
+    // The popover IDL attribute must reflect the popover attribute, limited to only known values.
+    auto value = get_attribute(HTML::AttributeNames::popover);
+
+    if (!value.has_value())
+        return {};
+
+    if (value.value().is_empty() || value.value().equals_ignoring_ascii_case("auto"sv))
+        return "auto"_string;
+
+    return "manual"_string;
+}
+
+// https://html.spec.whatwg.org/multipage/popover.html#dom-popover
+WebIDL::ExceptionOr<void> HTMLElement::set_popover(Optional<String> value)
+{
+    // FIXME: This should probably be `Reflect` in the IDL.
+    // The popover IDL attribute must reflect the popover attribute, limited to only known values.
+    if (value.has_value())
+        return set_attribute(HTML::AttributeNames::popover, value.release_value());
+
+    remove_attribute(HTML::AttributeNames::popover);
+    return {};
+}
+
 void HTMLElement::did_receive_focus()
 {
     if (m_content_editable_state != ContentEditableState::True)

Userland/Libraries/LibWeb/HTML/HTMLElement.h

@@ -77,6 +77,9 @@ class HTMLElement
 
     WebIDL::ExceptionOr<JS::NonnullGCPtr<ElementInternals>> attach_internals();
 
+    WebIDL::ExceptionOr<void> set_popover(Optional<String> value);
+    Optional<String> popover() const;
+
 protected:
     HTMLElement(DOM::Document&, DOM::QualifiedName);
 

Userland/Libraries/LibWeb/HTML/HTMLElement.idl

@@ -35,7 +35,7 @@ interface HTMLElement : Element {
     [FIXME] undefined showPopover();
     [FIXME] undefined hidePopover();
     [FIXME] boolean togglePopover(optional boolean force);
-    [FIXME, CEReactions] attribute DOMString? popover;
+    [CEReactions] attribute DOMString? popover;
 
     // https://drafts.csswg.org/cssom-view/#extensions-to-the-htmlelement-interface
     readonly attribute Element? offsetParent;