Bug 1675349: Forbid elements representing blocked plugins from accepting focus r=jmathies

As part of the fallback for unsupported plugin elements, we deny them focus.  Since elements can be programmatically changed, and object/embed elements can change internal "type" by updating their data/src attributes, and because that type is decided asynchronously, we may need to give up focus if our element has it.

Differential Revision: https://phabricator.services.mozilla.com/D95903

dom/html/HTMLObjectElement.cpp

@@ -289,6 +289,17 @@ bool HTMLObjectElement::IsHTMLFocusable(bool aWithMouse, bool* aIsFocusable,
     return false;
   }
 
+  // If we have decided that this is a blocked plugin then do not allow focus.
+  if ((Type() == eType_Null) &&
+      (PluginFallbackType() == eFallbackBlockAllPlugins)) {
+    if (aTabIndex) {
+      *aTabIndex = -1;
+    }
+
+    *aIsFocusable = false;
+    return false;
+  }
+
   const nsAttrValue* attrVal = mAttrs.GetAttr(nsGkAtoms::tabindex);
   bool isFocusable = attrVal && attrVal->Type() == nsAttrValue::eInteger;