--- a/layout/base/MobileViewportManager.cpp
+++ b/layout/base/MobileViewportManager.cpp
@@ -116,19 +116,20 @@ mozilla::CSSToScreenScale MobileViewport
     const mozilla::ScreenIntSize& aDisplaySize,
     const mozilla::CSSSize& aViewportOrContentSize) const {
   CSSToScreenScale intrinsicScale =
       MaxScaleRatio(ScreenSize(aDisplaySize), aViewportOrContentSize);
   MVM_LOG("%p: Intrinsic computed zoom is %f\n", this, intrinsicScale.scale);
   return ClampZoom(intrinsicScale, aViewportInfo);
 }
 
-void MobileViewportManager::RequestReflow() {
-  MVM_LOG("%p: got a reflow request\n", this);
-  RefreshViewportSize(false);
+void MobileViewportManager::RequestReflow(bool aForceAdjustResolution) {
+  MVM_LOG("%p: got a reflow request with force resolution: %d\n", this,
+          aForceAdjustResolution);
+  RefreshViewportSize(aForceAdjustResolution);
 }
 
 void MobileViewportManager::ResolutionUpdated() {
   MVM_LOG("%p: resolution updated\n", this);
 
   if (!mPresShell) {
     return;
   }
@@ -501,17 +502,18 @@ void MobileViewportManager::RefreshViewp
     return;
   }
 
   // If it's the first-paint or the viewport changed, we need to update
   // various APZ properties (the zoom and some things that might depend on it)
   MVM_LOG("%p: Updating properties because %d || %d\n", this, mIsFirstPaint,
           mMobileViewportSize != viewport);
 
-  if (nsLayoutUtils::AllowZoomingForDocument(mDocument)) {
+  if (aForceAdjustResolution ||
+      nsLayoutUtils::AllowZoomingForDocument(mDocument)) {
     UpdateResolution(viewportInfo, displaySize, viewport,
                      displayWidthChangeRatio, UpdateType::ViewportSize);
   } else {
     // Even without zoom, we need to update that the visual viewport size
     // has changed.
     RefreshVisualViewportSize();
   }
   if (gfxPlatform::AsyncPanZoomEnabled()) {

--- a/layout/base/MobileViewportManager.h
+++ b/layout/base/MobileViewportManager.h
@@ -55,17 +55,17 @@ class MobileViewportManager final : publ
   float ComputeIntrinsicResolution() const;
 
  private:
   void SetRestoreResolution(float aResolution);
 
  public:
   /* Notify the MobileViewportManager that a reflow was requested in the
    * presShell.*/
-  void RequestReflow();
+  void RequestReflow(bool aForceAdjustResolution);
 
   /* Notify the MobileViewportManager that the resolution on the presShell was
    * updated, and the visual viewport size needs to be updated. */
   void ResolutionUpdated();
 
   /* Called to compute the initial viewport on page load or before-first-paint,
    * whichever happens first. Also called directly if we are created after the
    * presShell is initialized. */

--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -1834,18 +1834,20 @@ nsresult PresShell::ResizeReflow(nscoord
     // If we have a ZoomConstraintsClient and the available screen area
     // changed, then we might need to disable double-tap-to-zoom, so notify
     // the ZCC to update itself.
     mZoomConstraintsClient->ScreenSizeChanged();
   }
   if (mMobileViewportManager) {
     // If we have a mobile viewport manager, request a reflow from it. It can
     // recompute the final CSS viewport and trigger a call to
-    // ResizeReflowIgnoreOverride if it changed.
-    mMobileViewportManager->RequestReflow();
+    // ResizeReflowIgnoreOverride if it changed. We don't force adjusting
+    // of resolution, because that is only necessary when we are destroying
+    // the MVM.
+    mMobileViewportManager->RequestReflow(false);
     return NS_OK;
   }
 
   return ResizeReflowIgnoreOverride(aWidth, aHeight, aOldWidth, aOldHeight,
                                     aOptions);
 }
 
 nsresult PresShell::ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight,
@@ -10522,18 +10524,29 @@ void PresShell::UpdateViewportOverridden
         mMobileViewportManager->SetInitialViewport();
       }
     }
     return;
   }
 
   MOZ_ASSERT(mMobileViewportManager,
              "Shouldn't reach this without a MobileViewportManager.");
-  mMobileViewportManager->Destroy();
-  mMobileViewportManager = nullptr;
+  // Before we get rid of our MVM, ask it to update the viewport while
+  // forcing resolution, which will undo any scaling it might have imposed.
+  // To do this correctly, we need to first null out mMobileViewportManager,
+  // because during reflow we will check PresShell::GetIsViewportOverriden(),
+  // which uses that value as a signifier.
+  RefPtr<MobileViewportManager> oldMVM;
+  mMobileViewportManager.swap(oldMVM);
+
+  oldMVM->RequestReflow(true);
+  ResetVisualViewportSize();
+
+  oldMVM->Destroy();
+  oldMVM = nullptr;
 
   if (aAfterInitialization) {
     // Force a reflow to our correct size by going back to the docShell
     // and asking it to reassert its size. This is necessary because
     // everything underneath the docShell, like the ViewManager, has been
     // altered by the MobileViewportManager in an irreversible way.
     nsDocShell* docShell =
         static_cast<nsDocShell*>(GetPresContext()->GetDocShell());