--- a/layout/base/AccessibleCaret.cpp
+++ b/layout/base/AccessibleCaret.cpp
@@ -2,16 +2,17 @@
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "AccessibleCaret.h"
 
 #include "AccessibleCaretLogger.h"
+#include "mozilla/FloatingPoint.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/ToString.h"
 #include "nsCanvasFrame.h"
 #include "nsCaret.h"
 #include "nsDOMTokenList.h"
 #include "nsIFrame.h"
 
 namespace mozilla {
@@ -111,16 +112,17 @@ AccessibleCaret::SetAppearance(Appearanc
   AC_LOG("%s: %s -> %s", __FUNCTION__, ToString(mAppearance).c_str(),
          ToString(aAppearance).c_str());
 
   mAppearance = aAppearance;
 
   // Need to reset rect since the cached rect will be compared in SetPosition.
   if (mAppearance == Appearance::None) {
     mImaginaryCaretRect = nsRect();
+    mZoomLevel = 0.0f;
   }
 }
 
 void
 AccessibleCaret::SetSelectionBarEnabled(bool aEnabled)
 {
   if (mSelectionBarEnabled == aEnabled) {
     return;
@@ -250,94 +252,97 @@ AccessibleCaret::SetPosition(nsIFrame* a
     nsCaret::GetGeometryForFrame(aFrame, aOffset, nullptr);
 
   imaginaryCaretRectInFrame =
     nsLayoutUtils::ClampRectToScrollFrames(aFrame, imaginaryCaretRectInFrame);
 
   if (imaginaryCaretRectInFrame.IsEmpty()) {
     // Don't bother to set the caret position since it's invisible.
     mImaginaryCaretRect = nsRect();
+    mZoomLevel = 0.0f;
     return PositionChangedResult::Invisible;
   }
 
   nsRect imaginaryCaretRect = imaginaryCaretRectInFrame;
   nsLayoutUtils::TransformRect(aFrame, RootFrame(), imaginaryCaretRect);
+  float zoomLevel = GetZoomLevel();
 
-  if (imaginaryCaretRect.IsEqualEdges(mImaginaryCaretRect)) {
+  if (imaginaryCaretRect.IsEqualEdges(mImaginaryCaretRect) &&
+      FuzzyEqualsMultiplicative(zoomLevel, mZoomLevel)) {
     return PositionChangedResult::NotChanged;
   }
 
   mImaginaryCaretRect = imaginaryCaretRect;
+  mZoomLevel = zoomLevel;
 
   // SetCaretElementStyle() and SetSelectionBarElementStyle() require the
   // input rect relative to container frame.
   nsRect imaginaryCaretRectInContainerFrame = imaginaryCaretRectInFrame;
   nsLayoutUtils::TransformRect(aFrame, CustomContentContainerFrame(),
                                imaginaryCaretRectInContainerFrame);
-  SetCaretElementStyle(imaginaryCaretRectInContainerFrame);
-  SetSelectionBarElementStyle(imaginaryCaretRectInContainerFrame);
+  SetCaretElementStyle(imaginaryCaretRectInContainerFrame, mZoomLevel);
+  SetSelectionBarElementStyle(imaginaryCaretRectInContainerFrame, mZoomLevel);
 
   return PositionChangedResult::Changed;
 }
 
 nsIFrame*
 AccessibleCaret::CustomContentContainerFrame() const
 {
   nsCanvasFrame* canvasFrame = mPresShell->GetCanvasFrame();
   Element* container = canvasFrame->GetCustomContentContainer();
   nsIFrame* containerFrame = container->GetPrimaryFrame();
   return containerFrame;
 }
 
 void
-AccessibleCaret::SetCaretElementStyle(const nsRect& aRect)
+AccessibleCaret::SetCaretElementStyle(const nsRect& aRect, float aZoomLevel)
 {
   nsPoint position = CaretElementPosition(aRect);
   nsAutoString styleStr;
   styleStr.AppendPrintf("left: %dpx; top: %dpx; padding-top: %dpx;",
                         nsPresContext::AppUnitsToIntCSSPixels(position.x),
                         nsPresContext::AppUnitsToIntCSSPixels(position.y),
                         nsPresContext::AppUnitsToIntCSSPixels(aRect.height));
 
-  float zoomLevel = GetZoomLevel();
   styleStr.AppendPrintf(" width: %.2fpx; height: %.2fpx; margin-left: %.2fpx",
-                        sWidth / zoomLevel,
-                        sHeight / zoomLevel,
-                        sMarginLeft / zoomLevel);
+                        sWidth / aZoomLevel,
+                        sHeight / aZoomLevel,
+                        sMarginLeft / aZoomLevel);
 
   ErrorResult rv;
   CaretElement()->SetAttribute(NS_LITERAL_STRING("style"), styleStr, rv);
   MOZ_ASSERT(!rv.Failed());
 
   AC_LOG("Set caret style: %s", NS_ConvertUTF16toUTF8(styleStr).get());
 }
 
 void
-AccessibleCaret::SetSelectionBarElementStyle(const nsRect& aRect)
+AccessibleCaret::SetSelectionBarElementStyle(const nsRect& aRect,
+                                             float aZoomLevel)
 {
   int32_t height = nsPresContext::AppUnitsToIntCSSPixels(aRect.height);
   nsAutoString barStyleStr;
   barStyleStr.AppendPrintf("margin-top: -%dpx; height: %dpx;",
                            height, height);
 
-  float zoomLevel = GetZoomLevel();
-  barStyleStr.AppendPrintf(" width: %.2fpx;", sBarWidth / zoomLevel);
+  barStyleStr.AppendPrintf(" width: %.2fpx;", sBarWidth / aZoomLevel);
 
   ErrorResult rv;
   SelectionBarElement()->SetAttribute(NS_LITERAL_STRING("style"), barStyleStr, rv);
   MOZ_ASSERT(!rv.Failed());
 
   AC_LOG("Set bar style: %s", NS_ConvertUTF16toUTF8(barStyleStr).get());
 }
 
 float
 AccessibleCaret::GetZoomLevel()
 {
   // Full zoom on desktop.
   float fullZoom = mPresShell->GetPresContext()->GetFullZoom();
 
-  // Pinch-zoom on B2G.
+  // Pinch-zoom on B2G or fennec.
   float resolution = mPresShell->GetCumulativeResolution();
 
   return fullZoom * resolution;
 }
 
 } // namespace mozilla

--- a/layout/base/AccessibleCaret.h
+++ b/layout/base/AccessibleCaret.h
@@ -99,17 +99,17 @@ public:
   // Visual Spec" in bug 921965.
   virtual void SetSelectionBarEnabled(bool aEnabled);
 
   // This enumeration representing the result returned by SetPosition().
   enum class PositionChangedResult : uint8_t {
     // Position is not changed.
     NotChanged,
 
-    // Position is changed.
+    // Position or zoom level is changed.
     Changed,
 
     // Position is out of scroll port.
     Invisible
   };
 
   friend std::ostream& operator<<(std::ostream& aStream,
                                   const PositionChangedResult& aResult);
@@ -133,18 +133,18 @@ public:
   // Element for 'Intersects' test. Container of image and bar elements.
   dom::Element* CaretElement() const
   {
     return mCaretElementHolder->GetContentNode();
   }
 
 protected:
   // Argument aRect should be relative to CustomContentContainerFrame().
-  void SetCaretElementStyle(const nsRect& aRect);
-  void SetSelectionBarElementStyle(const nsRect& aRect);
+  void SetCaretElementStyle(const nsRect& aRect, float aZoomLevel);
+  void SetSelectionBarElementStyle(const nsRect& aRect, float aZoomLevel);
 
   // Get current zoom level.
   float GetZoomLevel();
 
   // Element which contains the caret image for 'Contains' test.
   dom::Element* CaretImageElement() const
   {
     return CaretElement()->GetFirstElementChild();
@@ -205,16 +205,19 @@ protected:
   // outlive mPresShell.
   nsIPresShell* MOZ_NON_OWNING_REF const mPresShell = nullptr;
 
   RefPtr<dom::AnonymousContent> mCaretElementHolder;
 
   // mImaginaryCaretRect is relative to root frame.
   nsRect mImaginaryCaretRect;
 
+  // Cache current zoom level to determine whether position is changed.
+  float mZoomLevel = 0.0f;
+
   // A no-op touch-start listener which prevents APZ from panning when dragging
   // the caret.
   RefPtr<DummyTouchListener> mDummyTouchListener{new DummyTouchListener()};
 
   // Static class variables
   static float sWidth;
   static float sHeight;
   static float sMarginLeft;