--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -792,17 +792,17 @@ nsDOMWindowUtils::SendWheelEvent(float a
   }
 
   WidgetWheelEvent wheelEvent(true, eWheel, widget);
   wheelEvent.mModifiers = nsContentUtils::GetWidgetModifiers(aModifiers);
   wheelEvent.mDeltaX = aDeltaX;
   wheelEvent.mDeltaY = aDeltaY;
   wheelEvent.mDeltaZ = aDeltaZ;
   wheelEvent.mDeltaMode = aDeltaMode;
-  wheelEvent.isMomentum =
+  wheelEvent.mIsMomentum =
     (aOptions & WHEEL_EVENT_CAUSED_BY_MOMENTUM) != 0;
   wheelEvent.mIsNoLineOrPageDelta =
     (aOptions & WHEEL_EVENT_CAUSED_BY_NO_LINE_OR_PAGE_DELTA_DEVICE) != 0;
   wheelEvent.mCustomizedByUserPrefs =
     (aOptions & WHEEL_EVENT_CUSTOMIZED_BY_USER_PREFS) != 0;
   wheelEvent.lineOrPageDeltaX = aLineOrPageDeltaX;
   wheelEvent.lineOrPageDeltaY = aLineOrPageDeltaY;
   wheelEvent.widget = widget;

--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -2621,18 +2621,18 @@ EventStateManager::DoScrollText(nsIScrol
     case WidgetWheelEvent::SCROLL_SMOOTHLY:
       mode = nsIScrollableFrame::SMOOTH;
       break;
     default:
       MOZ_CRASH("Invalid scrollType value comes");
   }
 
   nsIScrollableFrame::ScrollMomentum momentum =
-    aEvent->isMomentum ? nsIScrollableFrame::SYNTHESIZED_MOMENTUM_EVENT
-                       : nsIScrollableFrame::NOT_MOMENTUM;
+    aEvent->mIsMomentum ? nsIScrollableFrame::SYNTHESIZED_MOMENTUM_EVENT
+                        : nsIScrollableFrame::NOT_MOMENTUM;
 
   nsIntPoint overflow;
   aScrollableFrame->ScrollBy(actualDevPixelScrollAmount,
                              nsIScrollableFrame::DEVICE_PIXELS,
                              mode, &overflow, origin, momentum, snapMode);
 
   if (!scrollFrameWeak.IsAlive()) {
     // If the scroll causes changing the layout, we can think that the event
@@ -5691,17 +5691,17 @@ EventStateManager::WheelPrefs::ComputeAc
     (Abs(aEvent->mDeltaX) > Abs(aEvent->mDeltaY) &&
      Abs(aEvent->mDeltaX) > Abs(aEvent->mDeltaZ));
   Action* actions = deltaXPreferred ? mOverriddenActionsX : mActions;
   if (actions[index] == ACTION_NONE || actions[index] == ACTION_SCROLL) {
     return actions[index];
   }
 
   // Momentum events shouldn't run special actions.
-  if (aEvent->isMomentum) {
+  if (aEvent->mIsMomentum) {
     // Use the default action.  Note that user might kill the wheel scrolling.
     Init(INDEX_DEFAULT);
     return (actions[INDEX_DEFAULT] == ACTION_SCROLL) ? ACTION_SCROLL :
                                                        ACTION_NONE;
   }
 
   return actions[index];
 }

--- a/widget/InputData.cpp
+++ b/widget/InputData.cpp
@@ -388,17 +388,17 @@ PanGestureInput::ToWidgetWheelEvent(nsIW
   wheelEvent.mTime = mTime;
   wheelEvent.mTimeStamp = mTimeStamp;
   wheelEvent.refPoint =
     RoundedToInt(ViewAs<LayoutDevicePixel>(mPanStartPoint,
       PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent));
   wheelEvent.buttons = 0;
   wheelEvent.mDeltaMode = nsIDOMWheelEvent::DOM_DELTA_PIXEL;
   wheelEvent.mMayHaveMomentum = true; // pan inputs may have momentum
-  wheelEvent.isMomentum = IsMomentum();
+  wheelEvent.mIsMomentum = IsMomentum();
   wheelEvent.lineOrPageDeltaX = mLineOrPageDeltaX;
   wheelEvent.lineOrPageDeltaY = mLineOrPageDeltaY;
   wheelEvent.mDeltaX = mPanDisplacement.x;
   wheelEvent.mDeltaY = mPanDisplacement.y;
   wheelEvent.mFlags.mHandledByAPZ = mHandledByAPZ;
   return wheelEvent;
 }
 
@@ -466,17 +466,17 @@ ScrollWheelInput::ScrollWheelInput(const
   , mDeltaX(aWheelEvent.mDeltaX)
   , mDeltaY(aWheelEvent.mDeltaY)
   , mLineOrPageDeltaX(aWheelEvent.lineOrPageDeltaX)
   , mLineOrPageDeltaY(aWheelEvent.lineOrPageDeltaY)
   , mScrollSeriesNumber(0)
   , mUserDeltaMultiplierX(1.0)
   , mUserDeltaMultiplierY(1.0)
   , mMayHaveMomentum(aWheelEvent.mMayHaveMomentum)
-  , mIsMomentum(aWheelEvent.isMomentum)
+  , mIsMomentum(aWheelEvent.mIsMomentum)
   , mAllowToOverrideSystemScrollSpeed(
       aWheelEvent.mAllowToOverrideSystemScrollSpeed)
 {
   mOrigin =
     ScreenPoint(ViewAs<ScreenPixel>(aWheelEvent.refPoint,
       PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent));
 }
 
@@ -488,17 +488,17 @@ ScrollWheelInput::ToWidgetWheelEvent(nsI
   wheelEvent.mTime = mTime;
   wheelEvent.mTimeStamp = mTimeStamp;
   wheelEvent.refPoint =
     RoundedToInt(ViewAs<LayoutDevicePixel>(mOrigin,
       PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent));
   wheelEvent.buttons = 0;
   wheelEvent.mDeltaMode = DeltaModeForDeltaType(mDeltaType);
   wheelEvent.mMayHaveMomentum = mMayHaveMomentum;
-  wheelEvent.isMomentum = mIsMomentum;
+  wheelEvent.mIsMomentum = mIsMomentum;
   wheelEvent.mDeltaX = mDeltaX;
   wheelEvent.mDeltaY = mDeltaY;
   wheelEvent.lineOrPageDeltaX = mLineOrPageDeltaX;
   wheelEvent.lineOrPageDeltaY = mLineOrPageDeltaY;
   wheelEvent.mAllowToOverrideSystemScrollSpeed =
     mAllowToOverrideSystemScrollSpeed;
   wheelEvent.mFlags.mHandledByAPZ = mHandledByAPZ;
   return wheelEvent;

--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -448,17 +448,17 @@ private:
   friend class mozilla::dom::PBrowserChild;
 
   WidgetWheelEvent()
     : mDeltaX(0.0)
     , mDeltaY(0.0)
     , mDeltaZ(0.0)
     , mDeltaMode(nsIDOMWheelEvent::DOM_DELTA_PIXEL)
     , mCustomizedByUserPrefs(false)
-    , isMomentum(false)
+    , mIsMomentum(false)
     , mIsNoLineOrPageDelta(false)
     , lineOrPageDeltaX(0)
     , lineOrPageDeltaY(0)
     , scrollType(SCROLL_DEFAULT)
     , overflowDeltaX(0.0)
     , overflowDeltaY(0.0)
     , mViewPortIsOverscrolled(false)
     , mCanTriggerSwipe(false)
@@ -472,17 +472,17 @@ public:
   WidgetWheelEvent(bool aIsTrusted, EventMessage aMessage, nsIWidget* aWidget)
     : WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, eWheelEventClass)
     , mDeltaX(0.0)
     , mDeltaY(0.0)
     , mDeltaZ(0.0)
     , mDeltaMode(nsIDOMWheelEvent::DOM_DELTA_PIXEL)
     , mCustomizedByUserPrefs(false)
     , mMayHaveMomentum(false)
-    , isMomentum(false)
+    , mIsMomentum(false)
     , mIsNoLineOrPageDelta(false)
     , lineOrPageDeltaX(0)
     , lineOrPageDeltaY(0)
     , scrollType(SCROLL_DEFAULT)
     , overflowDeltaX(0.0)
     , overflowDeltaY(0.0)
     , mViewPortIsOverscrolled(false)
     , mCanTriggerSwipe(false)
@@ -526,17 +526,17 @@ public:
 
   // If the delta values are computed from prefs, this value is true.
   // Otherwise, i.e., they are computed from native events, false.
   bool mCustomizedByUserPrefs;
 
   // true if the momentum events directly tied to this event may follow it.
   bool mMayHaveMomentum;
   // true if the event is caused by momentum.
-  bool isMomentum;
+  bool mIsMomentum;
 
   // If device event handlers don't know when they should set lineOrPageDeltaX
   // and lineOrPageDeltaY, this is true.  Otherwise, false.
   // If mIsNoLineOrPageDelta is true, ESM will generate
   // eLegacyMouseLineOrPageScroll events when accumulated delta values reach
   // a line height.
   bool mIsNoLineOrPageDelta;
 
@@ -611,17 +611,17 @@ public:
     AssignMouseEventBaseData(aEvent, aCopyTargets);
 
     mDeltaX = aEvent.mDeltaX;
     mDeltaY = aEvent.mDeltaY;
     mDeltaZ = aEvent.mDeltaZ;
     mDeltaMode = aEvent.mDeltaMode;
     mCustomizedByUserPrefs = aEvent.mCustomizedByUserPrefs;
     mMayHaveMomentum = aEvent.mMayHaveMomentum;
-    isMomentum = aEvent.isMomentum;
+    mIsMomentum = aEvent.mIsMomentum;
     mIsNoLineOrPageDelta = aEvent.mIsNoLineOrPageDelta;
     lineOrPageDeltaX = aEvent.lineOrPageDeltaX;
     lineOrPageDeltaY = aEvent.lineOrPageDeltaY;
     scrollType = aEvent.scrollType;
     overflowDeltaX = aEvent.overflowDeltaX;
     overflowDeltaY = aEvent.overflowDeltaY;
     mViewPortIsOverscrolled = aEvent.mViewPortIsOverscrolled;
     mCanTriggerSwipe = aEvent.mCanTriggerSwipe;

--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -5149,17 +5149,17 @@ PanGestureTypeForEvent(NSEvent* aEvent)
   [self convertCocoaMouseEvent:aMouseEvent toGeckoEvent:outWheelEvent];
 
   bool usePreciseDeltas = nsCocoaUtils::HasPreciseScrollingDeltas(aMouseEvent) &&
     Preferences::GetBool("mousewheel.enable_pixel_scrolling", true);
 
   outWheelEvent->mDeltaMode =
     usePreciseDeltas ? nsIDOMWheelEvent::DOM_DELTA_PIXEL
                      : nsIDOMWheelEvent::DOM_DELTA_LINE;
-  outWheelEvent->isMomentum = nsCocoaUtils::IsMomentumScrollEvent(aMouseEvent);
+  outWheelEvent->mIsMomentum = nsCocoaUtils::IsMomentumScrollEvent(aMouseEvent);
 }
 
 - (void) convertCocoaMouseEvent:(NSEvent*)aMouseEvent
                    toGeckoEvent:(WidgetInputEvent*)outGeckoEvent
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;
 
   NS_ASSERTION(outGeckoEvent, "convertCocoaMouseEvent:toGeckoEvent: requires non-null aoutGeckoEvent");

--- a/widget/nsGUIEventIPC.h
+++ b/widget/nsGUIEventIPC.h
@@ -161,17 +161,17 @@ struct ParamTraits<mozilla::WidgetWheelE
   {
     WriteParam(aMsg, static_cast<mozilla::WidgetMouseEventBase>(aParam));
     WriteParam(aMsg, aParam.mDeltaX);
     WriteParam(aMsg, aParam.mDeltaY);
     WriteParam(aMsg, aParam.mDeltaZ);
     WriteParam(aMsg, aParam.mDeltaMode);
     WriteParam(aMsg, aParam.mCustomizedByUserPrefs);
     WriteParam(aMsg, aParam.mMayHaveMomentum);
-    WriteParam(aMsg, aParam.isMomentum);
+    WriteParam(aMsg, aParam.mIsMomentum);
     WriteParam(aMsg, aParam.mIsNoLineOrPageDelta);
     WriteParam(aMsg, aParam.lineOrPageDeltaX);
     WriteParam(aMsg, aParam.lineOrPageDeltaY);
     WriteParam(aMsg, static_cast<int32_t>(aParam.scrollType));
     WriteParam(aMsg, aParam.overflowDeltaX);
     WriteParam(aMsg, aParam.overflowDeltaY);
     WriteParam(aMsg, aParam.mViewPortIsOverscrolled);
     WriteParam(aMsg, aParam.mCanTriggerSwipe);
@@ -185,17 +185,17 @@ struct ParamTraits<mozilla::WidgetWheelE
       ReadParam(aMsg, aIter,
                 static_cast<mozilla::WidgetMouseEventBase*>(aResult)) &&
       ReadParam(aMsg, aIter, &aResult->mDeltaX) &&
       ReadParam(aMsg, aIter, &aResult->mDeltaY) &&
       ReadParam(aMsg, aIter, &aResult->mDeltaZ) &&
       ReadParam(aMsg, aIter, &aResult->mDeltaMode) &&
       ReadParam(aMsg, aIter, &aResult->mCustomizedByUserPrefs) &&
       ReadParam(aMsg, aIter, &aResult->mMayHaveMomentum) &&
-      ReadParam(aMsg, aIter, &aResult->isMomentum) &&
+      ReadParam(aMsg, aIter, &aResult->mIsMomentum) &&
       ReadParam(aMsg, aIter, &aResult->mIsNoLineOrPageDelta) &&
       ReadParam(aMsg, aIter, &aResult->lineOrPageDeltaX) &&
       ReadParam(aMsg, aIter, &aResult->lineOrPageDeltaY) &&
       ReadParam(aMsg, aIter, &scrollType) &&
       ReadParam(aMsg, aIter, &aResult->overflowDeltaX) &&
       ReadParam(aMsg, aIter, &aResult->overflowDeltaY) &&
       ReadParam(aMsg, aIter, &aResult->mViewPortIsOverscrolled) &&
       ReadParam(aMsg, aIter, &aResult->mCanTriggerSwipe) &&