Bug 1811129 - Implement the new dialog initial focus algorithm r=emilio

The main changes of the new algorithm are

* Make the dialog focusing steps look at sequentially focusable elements instead of any focusable element.
* Make the dialog element itself get focus if it has the autofocus attribute set.
* Make the dialog element itself get focus as a fallback instead of focus being "reset" to the body element.

Spec PR (merged): whatwg/html#8199

Differential Revision: https://phabricator.services.mozilla.com/D181263

dom/base/FragmentOrElement.cpp

@@ -1036,9 +1036,14 @@ Element* nsIContent::GetFocusDelegate(bool aWithMouse,
     whereToLook = root;
   }
 
-  auto IsFocusable = [&](Element* aElement) {
+  auto IsFocusable = [&](Element* aElement) -> nsIFrame::Focusable {
     nsIFrame* frame = aElement->GetPrimaryFrame();
-    return frame && frame->IsFocusable(aWithMouse);
+
+    if (!frame) {
+      return {};
+    }
+
+    return frame->IsFocusable(aWithMouse);
   };
 
   Element* potentialFocus = nullptr;
@@ -1059,10 +1064,20 @@ Element* nsIContent::GetFocusDelegate(bool aWithMouse,
         // Found an autofocus candidate.
         return el;
       }
-    } else if (!potentialFocus && IsFocusable(el)) {
-      // This element could be the one if we can't find an
-      // autofocus candidate which has the precedence.
-      potentialFocus = el;
+    } else if (!potentialFocus) {
+      if (nsIFrame::Focusable focusable = IsFocusable(el)) {
+        if (IsHTMLElement(nsGkAtoms::dialog)) {
+          if (focusable.mTabIndex >= 0) {
+            // If focusTarget is a dialog element and descendant is sequentially
+            // focusable, then set focusableArea to descendant.
+            potentialFocus = el;
+          }
+        } else {
+          // This element could be the one if we can't find an
+          // autofocus candidate which has the precedence.
+          potentialFocus = el;
+        }
+      }
     }
 
     if (!autofocus && potentialFocus) {

dom/html/HTMLDialogElement.cpp

@@ -159,7 +159,9 @@ void HTMLDialogElement::FocusDialog() {
     doc->FlushPendingNotifications(FlushType::Frames);
   }
 
-  RefPtr<Element> control = GetFocusDelegate(false /* aWithMouse */);
+  RefPtr<Element> control = HasAttr(nsGkAtoms::autofocus)
+                                ? this
+                                : GetFocusDelegate(false /* aWithMouse */);
 
   // If there isn't one of those either, then let control be subject.
   if (!control) {
@@ -169,6 +171,8 @@ void HTMLDialogElement::FocusDialog() {
   FocusCandidate(*control, IsInTopLayer());
 }
 
+int32_t HTMLDialogElement::TabIndexDefault() { return 0; }
+
 void HTMLDialogElement::QueueCancelDialog() {
   // queues an element task on the user interaction task source
   OwnerDoc()

dom/html/HTMLDialogElement.h

@@ -49,6 +49,8 @@ class HTMLDialogElement final : public nsGenericHTMLElement {
 
   MOZ_CAN_RUN_SCRIPT_BOUNDARY void FocusDialog();
 
+  int32_t TabIndexDefault() override;
+
   nsString mReturnValue;
 
  protected: