Make fieldset match :enabled and :disabled again

In http://crrev.com/1122658 I made fieldset not disableable to get
around some event dispatching issues, and also made it stop matching
:disabled and :enabled. Due to feedback in
whatwg/html#5886 (comment) I am
making it match :disabled and :enabled again in this patch.

Bug: 588760
Change-Id: I3793c02353fe0ffbd45e94d90b79e5f7b61b97dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4627174
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Auto-Submit: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160137}

third_party/blink/renderer/core/css/selector_checker.cc

@@ -54,6 +54,7 @@
 #include "third_party/blink/renderer/core/frame/settings.h"
 #include "third_party/blink/renderer/core/fullscreen/fullscreen.h"
 #include "third_party/blink/renderer/core/html/custom/element_internals.h"
+#include "third_party/blink/renderer/core/html/forms/html_field_set_element.h"
 #include "third_party/blink/renderer/core/html/forms/html_form_control_element.h"
 #include "third_party/blink/renderer/core/html/forms/html_input_element.h"
 #include "third_party/blink/renderer/core/html/forms/html_option_element.h"
@@ -1619,6 +1620,16 @@ bool SelectorChecker::CheckPseudoClass(const SelectorCheckingContext& context,
     case CSSSelector::kPseudoDefault:
       return element.MatchesDefaultPseudoClass();
     case CSSSelector::kPseudoDisabled:
+      if (auto* fieldset = DynamicTo<HTMLFieldSetElement>(element)) {
+        if (RuntimeEnabledFeatures::
+                SendMouseEventsDisabledFormControlsEnabled()) {
+          // <fieldset> should never be considered disabled, but should still
+          // match the :enabled or :disabled pseudo-classes according to whether
+          // the attribute is set or not. See here for context:
+          // https://github.com/whatwg/html/issues/5886#issuecomment-1582410112
+          return fieldset->IsActuallyDisabled();
+        }
+      }
       return element.IsDisabledFormControl();
     case CSSSelector::kPseudoReadOnly:
       return element.MatchesReadOnlyPseudoClass();

third_party/blink/renderer/core/html/forms/html_field_set_element.cc

@@ -187,4 +187,15 @@ bool HTMLFieldSetElement::IsDisabledFormControl() const {
   return HTMLFormControlElement::IsDisabledFormControl();
 }
 
+// <fieldset> should never be considered disabled, but should still match the
+// :enabled or :disabled pseudo-classes according to whether the attribute is
+// set or not. See here for context:
+// https://github.com/whatwg/html/issues/5886#issuecomment-1582410112
+bool HTMLFieldSetElement::MatchesEnabledPseudoClass() const {
+  if (RuntimeEnabledFeatures::SendMouseEventsDisabledFormControlsEnabled()) {
+    return !IsActuallyDisabled();
+  }
+  return HTMLFormControlElement::MatchesEnabledPseudoClass();
+}
+
 }  // namespace blink

third_party/blink/renderer/core/html/forms/html_field_set_element.h

@@ -60,6 +60,7 @@ class CORE_EXPORT HTMLFieldSetElement final : public HTMLFormControlElement {
   bool AreAuthorShadowsAllowed() const override { return false; }
   bool IsSubmittableElement() override;
   bool AlwaysCreateUserAgentShadowRoot() const override { return false; }
+  bool MatchesEnabledPseudoClass() const final;
 
   Element* InvalidateDescendantDisabledStateAndFindFocusedOne(Element& base);
 };