--- a/gfx/ipc/GfxMessageUtils.h
+++ b/gfx/ipc/GfxMessageUtils.h
@@ -1293,26 +1293,28 @@ struct ParamTraits<mozilla::layers::Asyn
 
   static void Write(Message* aMsg, const paramType& aParam)
   {
     WriteParam(aMsg, aParam.mViewId);
     WriteParam(aMsg, aParam.mPresShellId);
     WriteParam(aMsg, aParam.mDragStartSequenceNumber);
     WriteParam(aMsg, aParam.mScrollbarDragOffset);
     WriteParam(aMsg, aParam.mScrollTrack);
+    WriteParam(aMsg, aParam.mScrollThumbLength);
     WriteParam(aMsg, aParam.mDirection);
   }
 
   static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
   {
     return (ReadParam(aMsg, aIter, &aResult->mViewId) &&
             ReadParam(aMsg, aIter, &aResult->mPresShellId) &&
             ReadParam(aMsg, aIter, &aResult->mDragStartSequenceNumber) &&
             ReadParam(aMsg, aIter, &aResult->mScrollbarDragOffset) &&
             ReadParam(aMsg, aIter, &aResult->mScrollTrack) &&
+            ReadParam(aMsg, aIter, &aResult->mScrollThumbLength) &&
             ReadParam(aMsg, aIter, &aResult->mDirection));
   }
 };
 
 template <>
 struct ParamTraits<mozilla::gfx::Glyph>
 {
   typedef mozilla::gfx::Glyph paramType;

--- a/gfx/layers/apz/src/AsyncDragMetrics.h
+++ b/gfx/layers/apz/src/AsyncDragMetrics.h
@@ -29,37 +29,41 @@ public:
   };
 
   // IPC constructor
   AsyncDragMetrics()
     : mViewId(0)
     , mPresShellId(0)
     , mDragStartSequenceNumber(0)
     , mScrollbarDragOffset(0)
+    , mScrollThumbLength(0)
     , mDirection(NONE)
   {}
 
   AsyncDragMetrics(const FrameMetrics::ViewID& aViewId,
                    uint32_t aPresShellId,
                    uint64_t aDragStartSequenceNumber,
                    CSSCoord aScrollbarDragOffset,
                    const CSSRect& aScrollTrack,
+                   CSSCoord aScrollThumbLength,
                    DragDirection aDirection)
     : mViewId(aViewId)
     , mPresShellId(aPresShellId)
     , mDragStartSequenceNumber(aDragStartSequenceNumber)
     , mScrollbarDragOffset(aScrollbarDragOffset)
     , mScrollTrack(aScrollTrack)
+    , mScrollThumbLength(aScrollThumbLength)
     , mDirection(aDirection)
   {}
 
   FrameMetrics::ViewID mViewId;
   uint32_t mPresShellId;
   uint64_t mDragStartSequenceNumber;
   CSSCoord mScrollbarDragOffset;
   CSSRect mScrollTrack;
+  CSSCoord mScrollThumbLength;
   DragDirection mDirection;
 };
 
 }
 }
 
 #endif

--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -922,19 +922,17 @@ nsEventStatus AsyncPanZoomController::Ha
   CSSRect cssCompositionBound = mFrameMetrics.CalculateCompositedRectInCssPixels();
 
   CSSCoord mousePosition = GetAxisStart(aDragMetrics.mDirection, scrollbarPoint) -
                         aDragMetrics.mScrollbarDragOffset -
                         GetAxisStart(aDragMetrics.mDirection, cssCompositionBound) -
                         GetAxisStart(aDragMetrics.mDirection, aDragMetrics.mScrollTrack);
 
   CSSCoord scrollMax = GetAxisLength(aDragMetrics.mDirection, aDragMetrics.mScrollTrack);
-  scrollMax -= node->GetScrollThumbLength() /
-               GetAxisScale(aDragMetrics.mDirection, mFrameMetrics.GetZoom()) *
-               mFrameMetrics.GetPresShellResolution();
+  scrollMax -= aDragMetrics.mScrollThumbLength;
 
   float scrollPercent = mousePosition / scrollMax;
 
   CSSCoord minScrollPosition =
     GetAxisStart(aDragMetrics.mDirection, mFrameMetrics.GetScrollableRect().TopLeft());
   CSSCoord maxScrollPosition =
     GetAxisLength(aDragMetrics.mDirection, mFrameMetrics.GetScrollableRect()) -
     GetAxisLength(aDragMetrics.mDirection, cssCompositionBound);

--- a/layout/xul/nsSliderFrame.cpp
+++ b/layout/xul/nsSliderFrame.cpp
@@ -1035,16 +1035,24 @@ nsSliderFrame::StartAPZDrag(WidgetGUIEve
     // The scrollbar mediator is not the scroll frame.
     // That means this scroll frame has a custom scrollbar mediator.
     // That's not supported in the APZ codepath.
     if (!scrollFrame) {
       return;
     }
   }
 
+  nsIFrame* thumbFrame = mFrames.FirstChild();
+  if (!thumbFrame) {
+    return;
+  }
+  bool isHorizontal = IsXULHorizontal();
+  nsSize thumbSize = thumbFrame->GetSize();
+  nscoord thumbLength = isHorizontal ? thumbSize.width : thumbSize.height;
+
   mozilla::layers::FrameMetrics::ViewID scrollTargetId;
   bool hasID = nsLayoutUtils::FindIDFor(scrollableContent, &scrollTargetId);
   bool hasAPZView = hasID && (scrollTargetId != layers::FrameMetrics::NULL_SCROLL_ID);
 
   if (!hasAPZView) {
     return;
   }
 
@@ -1060,18 +1068,20 @@ nsSliderFrame::StartAPZDrag(WidgetGUIEve
 
   nsIPresShell* shell = PresContext()->PresShell();
   uint64_t inputblockId = InputAPZContext::GetInputBlockId();
   uint32_t presShellId = shell->GetPresShellId();
   AsyncDragMetrics dragMetrics(scrollTargetId, presShellId, inputblockId,
                                NSAppUnitsToFloatPixels(mDragStart,
                                  float(AppUnitsPerCSSPixel())),
                                sliderTrackCSS,
-                               IsXULHorizontal() ? AsyncDragMetrics::HORIZONTAL :
-                                                   AsyncDragMetrics::VERTICAL);
+                               NSAppUnitsToFloatPixels(thumbLength,
+                                 float(AppUnitsPerCSSPixel())),
+                               isHorizontal ? AsyncDragMetrics::HORIZONTAL :
+                                              AsyncDragMetrics::VERTICAL);
 
   if (!nsLayoutUtils::HasDisplayPort(scrollableContent)) {
     return;
   }
 
   // It's important to set this before calling nsIWidget::StartAsyncScrollbarDrag(),
   // because in some configurations, that can call AsyncScrollbarDragRejected()
   // synchronously, which clears the flag (and we want it to stay cleared in