9 files changed, 88 insertions, 17 deletions
diff --git a/dom/base/DocumentOrShadowRoot.cpp b/dom/base/DocumentOrShadowRoot.cpp
index 8376ab97e..13ee3cb15 100644
--- a/dom/base/DocumentOrShadowRoot.cpp
+++ b/dom/base/DocumentOrShadowRoot.cpp
@@ -6,6 +6,8 @@
 
 #include "DocumentOrShadowRoot.h"
 #include "mozilla/dom/StyleSheetList.h"
+#include "nsDocument.h"
+#include "nsFocusManager.h"
 #include "ShadowRoot.h"
 #include "XULDocument.h"
 
@@ -100,5 +102,48 @@ DocumentOrShadowRoot::GetElementsByClassName(const nsAString& aClasses)
   return nsContentUtils::GetElementsByClassName(&AsNode(), aClasses);
 }
 
+nsIContent*
+DocumentOrShadowRoot::Retarget(nsIContent* aContent) const
+{
+  for (nsIContent* cur = aContent;
+       cur;
+       cur = cur->GetContainingShadowHost()) {
+    if (cur->SubtreeRoot() == &AsNode()) {
+      return cur;
+    }
+  }
+  return nullptr;
+}
+
+Element*
+DocumentOrShadowRoot::GetRetargetedFocusedElement()
+{
+  if (nsCOMPtr<nsPIDOMWindowOuter> window = AsNode().OwnerDoc()->GetWindow()) {
+    nsCOMPtr<nsPIDOMWindowOuter> focusedWindow;
+    nsIContent* focusedContent =
+      nsFocusManager::GetFocusedDescendant(window,
+                                           false,
+                                           getter_AddRefs(focusedWindow));
+    // be safe and make sure the element is from this document
+    if (focusedContent && focusedContent->OwnerDoc() == AsNode().OwnerDoc()) {
+      if (focusedContent->ChromeOnlyAccess()) {
+        focusedContent = focusedContent->FindFirstNonChromeOnlyAccessContent();
+      }
+
+      if (focusedContent) {
+        if (!nsDocument::IsWebComponentsEnabled(focusedContent)) {
+          return focusedContent->AsElement();
+        }
+
+        if (nsIContent* retarget = Retarget(focusedContent)) {
+          return retarget->AsElement();
+        }
+      }
+    }
+  }
+
+  return nullptr;
+}
+
 }
 }
diff --git a/dom/base/DocumentOrShadowRoot.h b/dom/base/DocumentOrShadowRoot.h
index 59be0c2a9..2092cd54f 100644
--- a/dom/base/DocumentOrShadowRoot.h
+++ b/dom/base/DocumentOrShadowRoot.h
@@ -116,6 +116,15 @@ public:
   ~DocumentOrShadowRoot() = default;
 
 protected:
+  nsIContent* Retarget(nsIContent* aContent) const;
+
+  /**
+   * If focused element's subtree root is this document or shadow root, return
+   * focused element, otherwise, get the shadow host recursively until the
+   * shadow host's subtree root is this document or shadow root.
+   */
+  Element* GetRetargetedFocusedElement();
+
   nsTArray<RefPtr<mozilla::StyleSheet>> mStyleSheets;
   RefPtr<mozilla::dom::StyleSheetList> mDOMStyleSheets;
 
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index 6fbe04d25..e33d3c63a 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -1138,6 +1138,15 @@ FragmentOrElement::GetAssignedSlot() const
   return slots ? slots->mAssignedSlot.get() : nullptr;
 }
 
+nsIContent*
+nsIContent::GetContainingShadowHost() const
+{
+  if (mozilla::dom::ShadowRoot* shadow = GetContainingShadow()) {
+    return shadow->GetHost();
+  }
+  return nullptr;
+}
+
 void
 FragmentOrElement::SetAssignedSlot(HTMLSlotElement* aSlot)
 {
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index 4aab9e435..c354e04c1 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -461,6 +461,12 @@ ShadowRoot::DistributeAllNodes()
   DistributionChanged();
 }
 
+Element*
+ShadowRoot::GetActiveElement()
+{
+  return GetRetargetedFocusedElement();
+}
+
 void
 ShadowRoot::GetInnerHTML(nsAString& aInnerHTML)
 {
diff --git a/dom/base/ShadowRoot.h b/dom/base/ShadowRoot.h
index eb772d49a..f83a6f774 100644
--- a/dom/base/ShadowRoot.h
+++ b/dom/base/ShadowRoot.h
@@ -120,6 +120,8 @@ public:
 
   // WebIDL methods.
   using mozilla::dom::DocumentOrShadowRoot::GetElementById;
+
+  Element* GetActiveElement();
   void GetInnerHTML(nsAString& aInnerHTML);
   void SetInnerHTML(const nsAString& aInnerHTML, ErrorResult& aError);
   void StyleSheetChanged();
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index a1b1408a5..40bfa97e2 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -3031,20 +3031,9 @@ Element*
 nsIDocument::GetActiveElement()
 {
   // Get the focused element.
-  if (nsCOMPtr<nsPIDOMWindowOuter> window = GetWindow()) {
-    nsCOMPtr<nsPIDOMWindowOuter> focusedWindow;
-    nsIContent* focusedContent =
-      nsFocusManager::GetFocusedDescendant(window, false,
-                                           getter_AddRefs(focusedWindow));
-    // be safe and make sure the element is from this document
-    if (focusedContent && focusedContent->OwnerDoc() == this) {
-      if (focusedContent->ChromeOnlyAccess()) {
-        focusedContent = focusedContent->FindFirstNonChromeOnlyAccessContent();
-      }
-      if (focusedContent) {
-        return focusedContent->AsElement();
-      }
-    }
+  Element* focusedElement = GetRetargetedFocusedElement();
+  if (focusedElement) {
+    return focusedElement;
   }
 
   // No focused element anywhere in this document.  Try to get the BODY.
diff --git a/dom/base/nsFocusManager.cpp b/dom/base/nsFocusManager.cpp
index c14087c8a..3fc9546e8 100644
--- a/dom/base/nsFocusManager.cpp
+++ b/dom/base/nsFocusManager.cpp
@@ -813,10 +813,11 @@ nsFocusManager::ContentRemoved(nsIDocument* aDocument, nsIContent* aContent)
   if (!window)
     return NS_OK;
 
-  // if the content is currently focused in the window, or is an ancestor
-  // of the currently focused element, reset the focus within that window.
+  // if the content is currently focused in the window, or is an
+  // shadow-including inclusive ancestor of the currently focused element,
+  // reset the focus within that window.
   nsIContent* content = window->GetFocusedNode();
-  if (content && nsContentUtils::ContentIsDescendantOf(content, aContent)) {
+  if (content && nsContentUtils::ContentIsHostIncludingDescendantOf(content, aContent)) {
     bool shouldShowFocusRing = window->ShouldShowFocusRing();
     window->SetFocusedNode(nullptr);
 
diff --git a/dom/base/nsIContent.h b/dom/base/nsIContent.h
index ce0a4e596..101d5e931 100644
--- a/dom/base/nsIContent.h
+++ b/dom/base/nsIContent.h
@@ -697,6 +697,14 @@ public:
   virtual mozilla::dom::ShadowRoot *GetContainingShadow() const = 0;
 
   /**
+   * Gets the shadow host if this content is in a shadow tree. That is, the host
+   * of |GetContainingShadow|, if its not null.
+   *
+   * @return The shadow host, if this is in shadow tree, or null.
+   */
+  nsIContent* GetContainingShadowHost() const;
+
+  /**
    * Gets the assigned slot associated with this content.
    *
    * @return The assigned slot element or null.
diff --git a/dom/webidl/ShadowRoot.webidl b/dom/webidl/ShadowRoot.webidl
index 47e6cf5ec..81b50e58b 100644
--- a/dom/webidl/ShadowRoot.webidl
+++ b/dom/webidl/ShadowRoot.webidl
@@ -20,6 +20,8 @@ enum ShadowRootMode {
 [Func="nsDocument::IsWebComponentsEnabled"]
 interface ShadowRoot : DocumentFragment
 {
+  readonly attribute Element? activeElement;
+
   // Shadow DOM v1
   readonly attribute ShadowRootMode mode;
   readonly attribute Element host;