Bug 1578448 - Make img.complete align with spec; r=bzbarsky

If the img has srcset attribute and the current request isn't complete or broken,
the img.complete should return false per spec,
https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-complete.

We should check srcset or src attribute, checking only mCurrentRequest isn't
enough given that updating mCurrentRequest happens async.

Differential Revision: https://phabricator.services.mozilla.com/D44678

dom/html/HTMLImageElement.cpp

@@ -164,11 +164,17 @@ bool HTMLImageElement::Draggable() const {
 }
 
 bool HTMLImageElement::Complete() {
-  if (!mCurrentRequest) {
-    return true;
+  // It is still not clear what value should img.complete return in various
+  // cases, see https://github.com/whatwg/html/issues/4884
+
+  if (!HasAttr(kNameSpaceID_None, nsGkAtoms::srcset)) {
+    nsAutoString src;
+    if (!GetAttr(kNameSpaceID_None, nsGkAtoms::src, src) || src.IsEmpty()) {
+      return true;
+    }
   }
 
-  if (mPendingRequest) {
+  if (!mCurrentRequest || mPendingRequest) {
     return false;
   }