Bug 1834519 - Don't throw when popovers/dialogs are in requested stat…

…e. r=emilio

Update to be in line with spec discussions at
whatwg/html#9142

Differential Revision: https://phabricator.services.mozilla.com/D178782

UltraBlame original commit: 39007d3d4f3eee8382115fb59e595de006ceca70

dom/html/HTMLDialogElement.cpp

@@ -71,7 +71,11 @@ void HTMLDialogElement::Close(
 
 void HTMLDialogElement::Show(ErrorResult& aError) {
   if (Open()) {
-    return;
+    if (!IsInTopLayer()) {
+      return;
+    }
+    return aError.ThrowInvalidStateError(
+        "Cannot call show() on an open modal dialog.");
   }
 
   if (IsPopoverOpen()) {
@@ -121,13 +125,16 @@ void HTMLDialogElement::UnbindFromTree(bool aNullParent) {
 }
 
 void HTMLDialogElement::ShowModal(ErrorResult& aError) {
-  if (!IsInComposedDoc()) {
-    return aError.ThrowInvalidStateError("Dialog element is not connected");
-  }
-
   if (Open()) {
+    if (IsInTopLayer()) {
+      return;
+    }
     return aError.ThrowInvalidStateError(
-        "Dialog element already has an 'open' attribute");
+        "Cannot call showModal() on an open non-modal dialog.");
+  }
+
+  if (!IsInComposedDoc()) {
+    return aError.ThrowInvalidStateError("Dialog element is not connected");
   }
 
   if (IsPopoverOpen()) {