Bug 1389314 Part3: Temporarily disable the input priority events when dnd is active. r=smaug.

MozReview-Commit-ID: AWfYmhjjsq0

dom/ipc/ContentParent.cpp

@@ -4451,6 +4451,14 @@ ContentParent::RecvSetOfflinePermission(const Principal& aPrincipal)
 void
 ContentParent::MaybeInvokeDragSession(TabParent* aParent)
 {
+  // dnd uses IPCBlob to transfer data to the content process and the IPC
+  // message is sent as normal priority. When sending input events with input
+  // priority, the message may be preempted by the later dnd events. To make
+  // sure the input events and the blob message are processed in time order
+  // on the content process, we temporarily send the input events with normal
+  // priority when there is an active dnd session.
+  SetInputPriorityEventEnabled(false);
+
   nsCOMPtr<nsIDragService> dragService =
     do_GetService("@mozilla.org/widget/dragservice;1");
   if (dragService && dragService->MaybeAddChildProcess(this)) {

dom/ipc/TabParent.cpp

@@ -3308,6 +3308,9 @@ TabParent::RecvInvokeDragSession(nsTArray<IPCDataTransfer>&& aTransfers,
       Unused << Manager()->AsContentParent()->SendEndDragSession(true, true,
                                                                  LayoutDeviceIntPoint(),
                                                                  0);
+      // Continue sending input events with input priority when stopping the dnd
+      // session.
+      Manager()->AsContentParent()->SetInputPriorityEventEnabled(true);
     }
     return IPC_OK();
   }

widget/nsBaseDragService.cpp

@@ -402,6 +402,9 @@ nsBaseDragService::EndDragSession(bool aDoneDrag, uint32_t aKeyModifiers)
                                                               mUserCancelled,
                                                               mEndDragPoint,
                                                               aKeyModifiers);
+    // Continue sending input events with input priority when stopping the dnd
+    // session.
+    mChildProcesses[i]->SetInputPriorityEventEnabled(true);
   }
   mChildProcesses.Clear();