Bug 1540150 - Make it impossible to enter the native drag-drop loop in test automation. r=NeilDeakin

Differential Revision: https://phabricator.services.mozilla.com/D25428

widget/nsBaseDragService.cpp

@@ -205,6 +205,23 @@ nsBaseDragService::InvokeDragSession(
     nsContentPolicyType aContentPolicyType = nsIContentPolicy::TYPE_OTHER) {
   AUTO_PROFILER_LABEL("nsBaseDragService::InvokeDragSession", OTHER);
 
+  // If you're hitting this, a test is causing the browser to attempt to enter
+  // the drag-drop native nested event loop, which will put the browser in a
+  // state that won't run tests properly until there's manual intervention
+  // to exit the drag-drop loop (either by moving the mouse or hitting escape),
+  // which can't be done from script since we're in the nested loop.
+  //
+  // The best way to avoid this is to catch the dragstart event on the item
+  // being dragged, and then to call preventDefault() and stopPropagating() on
+  // it. Alternatively, use EventUtils.synthesizeDragStart, which will do this
+  // for you.
+  if (XRE_IsParentProcess()) {
+    MOZ_ASSERT(
+        !xpc::IsInAutomation(),
+        "About to start drag-drop native loop on which will prevent later "
+        "tests from running properly.");
+  }
+
   NS_ENSURE_TRUE(aDOMNode, NS_ERROR_INVALID_ARG);
   NS_ENSURE_TRUE(mSuppressLevel == 0, NS_ERROR_FAILURE);