Bug 847453 - nsDOMWindowUtils::SendMouseEventCommon needs to set 'buttons' on the event. r=smaug.

dom/base/nsDOMWindowUtils.cpp

@@ -595,6 +595,26 @@ ToWidgetPoint(float aX, float aY, const nsPoint& aOffset,
                     NSToIntRound((aY*appPerCSS + aOffset.y)/appPerDev));
 }
 
+static inline int16_t
+GetButtonsFlagForButton(int32_t aButton)
+{
+  switch (aButton) {
+    case nsMouseEvent::eLeftButton:
+      return nsMouseEvent::eLeftButtonFlag;
+    case nsMouseEvent::eMiddleButton:
+      return nsMouseEvent::eMiddleButtonFlag;
+    case nsMouseEvent::eRightButton:
+      return nsMouseEvent::eRightButtonFlag;
+    case 4:
+      return nsMouseEvent::e4thButtonFlag;
+    case 5:
+      return nsMouseEvent::e5thButtonFlag;
+    default:
+      NS_ERROR("Button not known.");
+      return 0;
+  }
+}
+
 NS_IMETHODIMP
 nsDOMWindowUtils::SendMouseEventCommon(const nsAString& aType,
                                        float aX,
@@ -647,6 +667,7 @@ nsDOMWindowUtils::SendMouseEventCommon(const nsAString& aType,
                        nsMouseEvent::eContextMenuKey : nsMouseEvent::eNormal);
   event.modifiers = GetWidgetModifiers(aModifiers);
   event.button = aButton;
+  event.buttons = GetButtonsFlagForButton(aButton);
   event.widget = widget;
   event.pressure = aPressure;
   event.inputSource = aInputSourceArg;