--- a/dom/base/nsGlobalWindowOuter.cpp
+++ b/dom/base/nsGlobalWindowOuter.cpp
@@ -3243,52 +3243,65 @@ void nsGlobalWindowOuter::GetNameOuter(n
 void nsGlobalWindowOuter::SetNameOuter(const nsAString& aName,
                                        mozilla::ErrorResult& aError) {
   if (mDocShell) {
     aError = mDocShell->SetName(aName);
   }
 }
 
 // Helper functions used by many methods below.
-int32_t nsGlobalWindowOuter::DevToCSSIntPixels(int32_t px) {
-  if (!mDocShell) return px;  // assume 1:1
-
-  RefPtr<nsPresContext> presContext = mDocShell->GetPresContext();
-  if (!presContext) return px;
-
-  return presContext->DevPixelsToIntCSSPixels(px);
-}
-
-int32_t nsGlobalWindowOuter::CSSToDevIntPixels(int32_t px) {
-  if (!mDocShell) return px;  // assume 1:1
-
-  RefPtr<nsPresContext> presContext = mDocShell->GetPresContext();
-  if (!presContext) return px;
-
-  return presContext->CSSPixelsToDevPixels(px);
-}
-
-nsIntSize nsGlobalWindowOuter::DevToCSSIntPixels(nsIntSize px) {
-  if (!mDocShell) return px;  // assume 1:1
-
-  RefPtr<nsPresContext> presContext = mDocShell->GetPresContext();
-  if (!presContext) return px;
-
-  return nsIntSize(presContext->DevPixelsToIntCSSPixels(px.width),
-                   presContext->DevPixelsToIntCSSPixels(px.height));
-}
-
-nsIntSize nsGlobalWindowOuter::CSSToDevIntPixels(nsIntSize px) {
-  if (!mDocShell) return px;  // assume 1:1
-
-  RefPtr<nsPresContext> presContext = mDocShell->GetPresContext();
-  if (!presContext) return px;
-
-  return nsIntSize(presContext->CSSPixelsToDevPixels(px.width),
-                   presContext->CSSPixelsToDevPixels(px.height));
+int32_t nsGlobalWindowOuter::DevToCSSIntPixelsForBaseWindow(
+    int32_t aDevicePixels, nsIBaseWindow* aWindow) {
+  MOZ_ASSERT(aWindow);
+  double scale;
+  aWindow->GetUnscaledDevicePixelsPerCSSPixel(&scale);
+  double zoom = 1.0;
+  if (mDocShell && mDocShell->GetPresContext()) {
+    zoom = mDocShell->GetPresContext()->GetFullZoom();
+  }
+  return std::floor(aDevicePixels / scale / zoom + 0.5);
+}
+
+nsIntSize nsGlobalWindowOuter::DevToCSSIntPixelsForBaseWindow(
+    nsIntSize aDeviceSize, nsIBaseWindow* aWindow) {
+  MOZ_ASSERT(aWindow);
+  double scale;
+  aWindow->GetUnscaledDevicePixelsPerCSSPixel(&scale);
+  double zoom = 1.0;
+  if (mDocShell && mDocShell->GetPresContext()) {
+    zoom = mDocShell->GetPresContext()->GetFullZoom();
+  }
+  return nsIntSize::Round(
+      static_cast<float>(aDeviceSize.width / scale / zoom),
+      static_cast<float>(aDeviceSize.height / scale / zoom));
+}
+
+int32_t nsGlobalWindowOuter::CSSToDevIntPixelsForBaseWindow(
+    int32_t aCSSPixels, nsIBaseWindow* aWindow) {
+  MOZ_ASSERT(aWindow);
+  double scale;
+  aWindow->GetUnscaledDevicePixelsPerCSSPixel(&scale);
+  double zoom = 1.0;
+  if (mDocShell && mDocShell->GetPresContext()) {
+    zoom = mDocShell->GetPresContext()->GetFullZoom();
+  }
+  return std::floor(aCSSPixels * scale * zoom + 0.5);
+}
+
+nsIntSize nsGlobalWindowOuter::CSSToDevIntPixelsForBaseWindow(
+    nsIntSize aCSSSize, nsIBaseWindow* aWindow) {
+  MOZ_ASSERT(aWindow);
+  double scale;
+  aWindow->GetUnscaledDevicePixelsPerCSSPixel(&scale);
+  double zoom = 1.0;
+  if (mDocShell && mDocShell->GetPresContext()) {
+    zoom = mDocShell->GetPresContext()->GetFullZoom();
+  }
+  return nsIntSize::Round(static_cast<float>(aCSSSize.width * scale * zoom),
+                          static_cast<float>(aCSSSize.height * scale * zoom));
 }
 
 nsresult nsGlobalWindowOuter::GetInnerSize(CSSIntSize& aSize) {
   EnsureSizeAndPositionUpToDate();
 
   NS_ENSURE_STATE(mDocShell);
 
   RefPtr<nsPresContext> presContext = mDocShell->GetPresContext();
@@ -3355,17 +3368,18 @@ void nsGlobalWindowOuter::SetInnerWidthO
   }
 
   // Nothing has been overriden, so change the docshell itself.
   int32_t height = 0;
   int32_t unused = 0;
 
   nsCOMPtr<nsIBaseWindow> docShellAsWin(do_QueryInterface(mDocShell));
   docShellAsWin->GetSize(&unused, &height);
-  aError = SetDocShellWidthAndHeight(CSSToDevIntPixels(aInnerWidth), height);
+  aError = SetDocShellWidthAndHeight(
+      CSSToDevIntPixelsForBaseWindow(aInnerWidth, docShellAsWin), height);
 }
 
 int32_t nsGlobalWindowOuter::GetInnerHeightOuter(ErrorResult& aError) {
   CSSIntSize size;
   aError = GetInnerSize(size);
   return size.height;
 }
 
@@ -3400,17 +3414,18 @@ void nsGlobalWindowOuter::SetInnerHeight
   }
 
   // Nothing has been overriden, so change the docshell itself.
   int32_t height = 0;
   int32_t width = 0;
 
   nsCOMPtr<nsIBaseWindow> docShellAsWin(do_QueryInterface(mDocShell));
   docShellAsWin->GetSize(&width, &height);
-  aError = SetDocShellWidthAndHeight(width, CSSToDevIntPixels(aInnerHeight));
+  aError = SetDocShellWidthAndHeight(
+      width, CSSToDevIntPixelsForBaseWindow(aInnerHeight, docShellAsWin));
 }
 
 nsIntSize nsGlobalWindowOuter::GetOuterSize(CallerType aCallerType,
                                             ErrorResult& aError) {
   if (nsContentUtils::ResistFingerprinting(aCallerType)) {
     CSSIntSize size;
     aError = GetInnerSize(size);
     return nsIntSize(size.width, size.height);
@@ -3433,17 +3448,17 @@ nsIntSize nsGlobalWindowOuter::GetOuterS
   }
 
   nsIntSize sizeDevPixels;
   aError = treeOwnerAsWin->GetSize(&sizeDevPixels.width, &sizeDevPixels.height);
   if (aError.Failed()) {
     return nsIntSize();
   }
 
-  return DevToCSSIntPixels(sizeDevPixels);
+  return DevToCSSIntPixelsForBaseWindow(sizeDevPixels, treeOwnerAsWin);
 }
 
 int32_t nsGlobalWindowOuter::GetOuterWidthOuter(CallerType aCallerType,
                                                 ErrorResult& aError) {
   return GetOuterSize(aCallerType, aError).width;
 }
 
 int32_t nsGlobalWindowOuter::GetOuterHeightOuter(CallerType aCallerType,
@@ -3465,17 +3480,18 @@ void nsGlobalWindowOuter::SetOuterSize(i
                               aCallerType);
 
   int32_t width, height;
   aError = treeOwnerAsWin->GetSize(&width, &height);
   if (aError.Failed()) {
     return;
   }
 
-  int32_t lengthDevPixels = CSSToDevIntPixels(aLengthCSSPixels);
+  int32_t lengthDevPixels =
+      CSSToDevIntPixelsForBaseWindow(aLengthCSSPixels, treeOwnerAsWin);
   if (aIsWidth) {
     width = lengthDevPixels;
   } else {
     height = lengthDevPixels;
   }
   aError = treeOwnerAsWin->SetSize(width, height, true);
 
   CheckForDPIChange();
@@ -3690,17 +3706,17 @@ void nsGlobalWindowOuter::SetScreenXOute
 
   int32_t x, y;
   aError = treeOwnerAsWin->GetPosition(&x, &y);
   if (aError.Failed()) {
     return;
   }
 
   CheckSecurityLeftAndTop(&aScreenX, nullptr, aCallerType);
-  x = CSSToDevIntPixels(aScreenX);
+  x = CSSToDevIntPixelsForBaseWindow(aScreenX, treeOwnerAsWin);
 
   aError = treeOwnerAsWin->SetPosition(x, y);
 
   CheckForDPIChange();
 }
 
 int32_t nsGlobalWindowOuter::GetScreenYOuter(CallerType aCallerType,
                                              ErrorResult& aError) {
@@ -3718,17 +3734,17 @@ void nsGlobalWindowOuter::SetScreenYOute
 
   int32_t x, y;
   aError = treeOwnerAsWin->GetPosition(&x, &y);
   if (aError.Failed()) {
     return;
   }
 
   CheckSecurityLeftAndTop(nullptr, &aScreenY, aCallerType);
-  y = CSSToDevIntPixels(aScreenY);
+  y = CSSToDevIntPixelsForBaseWindow(aScreenY, treeOwnerAsWin);
 
   aError = treeOwnerAsWin->SetPosition(x, y);
 
   CheckForDPIChange();
 }
 
 // NOTE: Arguments to this function should have values scaled to
 // CSS pixels, not device pixels.
@@ -3805,32 +3821,33 @@ void nsGlobalWindowOuter::CheckSecurityL
     nsContentUtils::HidePopupsInDocument(mDoc);
 #endif
 
     if (nsGlobalWindowOuter* rootWindow =
             nsGlobalWindowOuter::Cast(GetPrivateRoot())) {
       rootWindow->FlushPendingNotifications(FlushType::Layout);
     }
 
-    nsCOMPtr<nsIBaseWindow> treeOwner = GetTreeOwnerWindow();
+    nsCOMPtr<nsIBaseWindow> treeOwnerAsWin = GetTreeOwnerWindow();
 
     RefPtr<nsScreen> screen = GetScreen();
 
-    if (treeOwner && screen) {
+    if (treeOwnerAsWin && screen) {
       int32_t winLeft, winTop, winWidth, winHeight;
 
       // Get the window size
-      treeOwner->GetPositionAndSize(&winLeft, &winTop, &winWidth, &winHeight);
+      treeOwnerAsWin->GetPositionAndSize(&winLeft, &winTop, &winWidth,
+                                         &winHeight);
 
       // convert those values to CSS pixels
       // XXX four separate retrievals of the prescontext
-      winLeft = DevToCSSIntPixels(winLeft);
-      winTop = DevToCSSIntPixels(winTop);
-      winWidth = DevToCSSIntPixels(winWidth);
-      winHeight = DevToCSSIntPixels(winHeight);
+      winLeft = DevToCSSIntPixelsForBaseWindow(winLeft, treeOwnerAsWin);
+      winTop = DevToCSSIntPixelsForBaseWindow(winTop, treeOwnerAsWin);
+      winWidth = DevToCSSIntPixelsForBaseWindow(winWidth, treeOwnerAsWin);
+      winHeight = DevToCSSIntPixelsForBaseWindow(winHeight, treeOwnerAsWin);
 
       // Get the screen dimensions
       // XXX This should use nsIScreenManager once it's fully fleshed out.
       int32_t screenLeft = screen->GetAvailLeft(IgnoreErrors());
       int32_t screenWidth = screen->GetAvailWidth(IgnoreErrors());
       int32_t screenHeight = screen->GetAvailHeight(IgnoreErrors());
 #if defined(XP_MACOSX)
       /* The mac's coordinate system is different from the assumed Windows'
@@ -5097,24 +5114,25 @@ void nsGlobalWindowOuter::MoveByOuter(in
 
   int32_t x, y;
   aError = treeOwnerAsWin->GetPosition(&x, &y);
   if (aError.Failed()) {
     return;
   }
 
   // mild abuse of a "size" object so we don't need more helper functions
-  nsIntSize cssPos(DevToCSSIntPixels(nsIntSize(x, y)));
+  nsIntSize cssPos(
+      DevToCSSIntPixelsForBaseWindow(nsIntSize(x, y), treeOwnerAsWin));
 
   cssPos.width += aXDif;
   cssPos.height += aYDif;
 
   CheckSecurityLeftAndTop(&cssPos.width, &cssPos.height, aCallerType);
 
-  nsIntSize newDevPos(CSSToDevIntPixels(cssPos));
+  nsIntSize newDevPos(CSSToDevIntPixelsForBaseWindow(cssPos, treeOwnerAsWin));
 
   aError = treeOwnerAsWin->SetPosition(newDevPos.width, newDevPos.height);
 
   CheckForDPIChange();
 }
 
 nsresult nsGlobalWindowOuter::MoveBy(int32_t aXDif, int32_t aYDif) {
   ErrorResult rv;
@@ -5152,17 +5170,17 @@ void nsGlobalWindowOuter::ResizeToOuter(
   if (!treeOwnerAsWin) {
     aError.Throw(NS_ERROR_FAILURE);
     return;
   }
 
   nsIntSize cssSize(aWidth, aHeight);
   CheckSecurityWidthAndHeight(&cssSize.width, &cssSize.height, aCallerType);
 
-  nsIntSize devSz(CSSToDevIntPixels(cssSize));
+  nsIntSize devSz(CSSToDevIntPixelsForBaseWindow(cssSize, treeOwnerAsWin));
 
   aError = treeOwnerAsWin->SetSize(devSz.width, devSz.height, true);
 
   CheckForDPIChange();
 }
 
 void nsGlobalWindowOuter::ResizeByOuter(int32_t aWidthDif, int32_t aHeightDif,
                                         CallerType aCallerType,
@@ -5207,24 +5225,25 @@ void nsGlobalWindowOuter::ResizeByOuter(
   if (aError.Failed()) {
     return;
   }
 
   // To do this correctly we have to convert what we got from GetSize
   // into CSS pixels, add the arguments, do the security check, and
   // then convert back to device pixels for the call to SetSize.
 
-  nsIntSize cssSize(DevToCSSIntPixels(nsIntSize(width, height)));
+  nsIntSize cssSize(
+      DevToCSSIntPixelsForBaseWindow(nsIntSize(width, height), treeOwnerAsWin));
 
   cssSize.width += aWidthDif;
   cssSize.height += aHeightDif;
 
   CheckSecurityWidthAndHeight(&cssSize.width, &cssSize.height, aCallerType);
 
-  nsIntSize newDevSize(CSSToDevIntPixels(cssSize));
+  nsIntSize newDevSize(CSSToDevIntPixelsForBaseWindow(cssSize, treeOwnerAsWin));
 
   aError = treeOwnerAsWin->SetSize(newDevSize.width, newDevSize.height, true);
 
   CheckForDPIChange();
 }
 
 void nsGlobalWindowOuter::SizeToContentOuter(CallerType aCallerType,
                                              ErrorResult& aError) {
@@ -5245,34 +5264,45 @@ void nsGlobalWindowOuter::SizeToContentO
   // viewer for a toplevel docshell.
   nsCOMPtr<nsIContentViewer> cv;
   mDocShell->GetContentViewer(getter_AddRefs(cv));
   if (!cv) {
     aError.Throw(NS_ERROR_FAILURE);
     return;
   }
 
-  int32_t width, height;
-  aError = cv->GetContentSize(&width, &height);
+  nsIntSize contentSize;
+  aError = cv->GetContentSize(&contentSize.width, &contentSize.height);
   if (aError.Failed()) {
     return;
   }
 
   // Make sure the new size is following the CheckSecurityWidthAndHeight
   // rules.
   nsCOMPtr<nsIDocShellTreeOwner> treeOwner = GetTreeOwner();
   if (!treeOwner) {
     aError.Throw(NS_ERROR_FAILURE);
     return;
   }
 
-  nsIntSize cssSize(DevToCSSIntPixels(nsIntSize(width, height)));
+  // Don't use DevToCSSIntPixelsForBaseWindow() nor
+  // CSSToDevIntPixelsForBaseWindow() here because contentSize is comes from
+  // nsIContentViewer::GetContentSize() and it's computed with nsPresContext so
+  // that we need to work with nsPresContext here too.
+  RefPtr<nsPresContext> presContext = cv->GetPresContext();
+  MOZ_ASSERT(
+      presContext,
+      "Should be non-nullptr if nsIContentViewer::GetContentSize() succeeded");
+  nsIntSize cssSize(presContext->DevPixelsToIntCSSPixels(contentSize.width),
+                    presContext->DevPixelsToIntCSSPixels(contentSize.height));
+
   CheckSecurityWidthAndHeight(&cssSize.width, &cssSize.height, aCallerType);
 
-  nsIntSize newDevSize(CSSToDevIntPixels(cssSize));
+  nsIntSize newDevSize(presContext->CSSPixelsToDevPixels(cssSize.width),
+                       presContext->CSSPixelsToDevPixels(cssSize.height));
 
   nsCOMPtr<nsIDocShell> docShell = mDocShell;
   aError =
       treeOwner->SizeShellTo(docShell, newDevSize.width, newDevSize.height);
 }
 
 already_AddRefed<nsPIWindowRoot> nsGlobalWindowOuter::GetTopWindowRoot() {
   nsPIDOMWindowOuter* piWin = GetPrivateRoot();

--- a/dom/base/nsGlobalWindowOuter.h
+++ b/dom/base/nsGlobalWindowOuter.h
@@ -895,22 +895,34 @@ class nsGlobalWindowOuter final : public
   bool IsInModalState();
 
   bool HasStorageAccess() const { return mHasStorageAccess; }
   void SetHasStorageAccess(bool aHasStorageAccess) {
     mHasStorageAccess = aHasStorageAccess;
   }
 
   // Convenience functions for the many methods that need to scale
-  // from device to CSS pixels or vice versa.  Note: if a presentation
-  // context is not available, they will assume a 1:1 ratio.
+  // from device to CSS pixels.  This computes it with cached scale in
+  // PresContext which may be not recent information of the widget.
+  // Note: if PresContext is not available, they will assume a 1:1 ratio.
   int32_t DevToCSSIntPixels(int32_t px);
-  int32_t CSSToDevIntPixels(int32_t px);
   nsIntSize DevToCSSIntPixels(nsIntSize px);
-  nsIntSize CSSToDevIntPixels(nsIntSize px);
+
+  // Convenience functions for the methods which call methods of nsIBaseWindow
+  // because it takes/returns device pixels.  Unfortunately, mPresContext may
+  // have older scale value for the corresponding widget.  Therefore, these
+  // helper methods convert between CSS pixels and device pixels with aWindow.
+  int32_t DevToCSSIntPixelsForBaseWindow(int32_t aDevicePixels,
+                                         nsIBaseWindow* aWindow);
+  nsIntSize DevToCSSIntPixelsForBaseWindow(nsIntSize aDeviceSize,
+                                           nsIBaseWindow* aWindow);
+  int32_t CSSToDevIntPixelsForBaseWindow(int32_t aCSSPixels,
+                                         nsIBaseWindow* aWindow);
+  nsIntSize CSSToDevIntPixelsForBaseWindow(nsIntSize aCSSSize,
+                                           nsIBaseWindow* aWindow);
 
   virtual void SetFocusedElement(mozilla::dom::Element* aElement,
                                  uint32_t aFocusMethod = 0,
                                  bool aNeedsFocus = false) override;
 
   virtual uint32_t GetFocusMethod() override;
 
   virtual bool ShouldShowFocusRing() override;