Implement ShadowRoot.delegatesFocus 2/4 (slide focus on mouse click)

Source/core/input/EventHandler.cpp

Index: Source/core/input/EventHandler.cpp
diff --git a/Source/core/input/EventHandler.cpp b/Source/core/input/EventHandler.cpp
index 6de87e31af73df1a1f2000de30e5dc1701f63e33..41003478ecf59c73c7289fca2080a26f331f6996 100644
--- a/Source/core/input/EventHandler.cpp
+++ b/Source/core/input/EventHandler.cpp
@@ -2071,6 +2071,8 @@ bool EventHandler::handleMouseFocus(const MouseEventWithHitTestResults& targeted
         // clear swallowEvent if the page already set it (e.g., by canceling
         // default behavior).
         if (element) {
+            if (slideFocusOnShadowHostIfNecessary(*element))
+                return true;
             if (!page->focusController().setFocusedElement(element, m_frame, WebFocusTypeMouse))
                 return true;
         } else {
@@ -2086,6 +2088,28 @@ bool EventHandler::handleMouseFocus(const MouseEventWithHitTestResults& targeted
     return false;
 }
 
+bool EventHandler::slideFocusOnShadowHostIfNecessary(const Element& element)
+{
+    if (element.shadowRoot() && element.shadowRoot()->delegatesFocus()) {
+        Document* doc = m_frame->document();
+        if (element.containsIncludingShadowDOM(doc->focusedElement())) {
+            // If the inner element is already focused, do nothing.
+            return true;
+        }
+
+        // If the host has a focusable inner element, focus it. Otherwise, the host takes focus.
+        Page* page = m_frame->page();
+        ASSERT(page);
+        Node* next = page->focusController().findFocusableNode(WebFocusTypeForward, *element.shadowRoot());
+        if (next && next->isElementNode() && element.containsIncludingShadowDOM(next)) {
+            // Use WebFocusTypeForward instead of WebFocusTypeMouse here to mean the focus has slided.
+            toElement(next)->focus(false, WebFocusTypeForward);
+            return true;
+        }
+    }
+    return false;
+}
+
 bool EventHandler::handleWheelEvent(const PlatformWheelEvent& event)
 {
 #define RETURN_WHEEL_EVENT_HANDLED() \