diff --git a/widget/cocoa/VibrancyManager.h b/widget/cocoa/VibrancyManager.h
index d7c96427bca2..19868e2d108d 100644
--- a/widget/cocoa/VibrancyManager.h
+++ b/widget/cocoa/VibrancyManager.h
@@ -73,6 +73,8 @@ class VibrancyManager {
 
   bool HasVibrantRegions() { return !mVibrantRegions.IsEmpty(); }
 
+  LayoutDeviceIntRegion GetUnionOfVibrantRegions() const;
+
   /**
    * Return the fill color that should be drawn on top of the cleared window
    * parts. Usually this would be drawn by -[NSVisualEffectView drawRect:].
diff --git a/widget/cocoa/VibrancyManager.mm b/widget/cocoa/VibrancyManager.mm
index 3922662f5b38..9a6fbaae3aaf 100644
--- a/widget/cocoa/VibrancyManager.mm
+++ b/widget/cocoa/VibrancyManager.mm
@@ -23,6 +23,14 @@ bool VibrancyManager::UpdateVibrantRegion(VibrancyType aType,
   });
 }
 
+LayoutDeviceIntRegion VibrancyManager::GetUnionOfVibrantRegions() const {
+  LayoutDeviceIntRegion result;
+  for (auto it = mVibrantRegions.ConstIter(); !it.Done(); it.Next()) {
+    result.OrWith(it.UserData()->Region());
+  }
+  return result;
+}
+
 @interface NSView (CurrentFillColor)
 - (NSColor*)_currentFillColor;
 @end
diff --git a/widget/cocoa/nsChildView.h b/widget/cocoa/nsChildView.h
index c4f34d11ea07..e505a6420f57 100644
--- a/widget/cocoa/nsChildView.h
+++ b/widget/cocoa/nsChildView.h
@@ -621,6 +621,8 @@ class nsChildView final : public nsBaseWidget {
   void UpdateVibrancy(const nsTArray<ThemeGeometry>& aThemeGeometries);
   mozilla::VibrancyManager& EnsureVibrancyManager();
 
+  void UpdateInternalOpaqueRegion();
+
   nsIWidget* GetWidgetForListenerEvents();
 
   struct SwipeInfo {
@@ -736,6 +738,9 @@ class nsChildView final : public nsBaseWidget {
 
   RefPtr<mozilla::CancelableRunnable> mUnsuspendAsyncCATransactionsRunnable;
 
+  // The widget's opaque region. Written on the main thread, read on any thread.
+  mozilla::DataMutex<mozilla::LayoutDeviceIntRegion> mOpaqueRegion;
+
   // This flag is only used when APZ is off. It indicates that the current pan
   // gesture was processed as a swipe. Sometimes the swipe animation can finish
   // before momentum events of the pan gesture have stopped firing, so this
diff --git a/widget/cocoa/nsChildView.mm b/widget/cocoa/nsChildView.mm
index ebe9879a7628..64039892a969 100644
--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -338,6 +338,7 @@ nsChildView::nsChildView()
       mIsDispatchPaint(false),
       mPluginFocused{false},
       mCompositingState("nsChildView::mCompositingState"),
+      mOpaqueRegion("nsChildView::mOpaqueRegion"),
       mCurrentPanGestureBelongsToSwipe{false} {}
 
 nsChildView::~nsChildView() {
@@ -613,6 +614,8 @@ void nsChildView::SetTransparencyMode(nsTransparencyMode aMode) {
     windowWidget->SetTransparencyMode(aMode);
   }
 
+  UpdateInternalOpaqueRegion();
+
   NS_OBJC_END_TRY_ABORT_BLOCK;
 }
 
@@ -2492,6 +2495,8 @@ void nsChildView::UpdateVibrancy(const nsTArray<ThemeGeometry>& aThemeGeometries
   changed |= vm.UpdateVibrantRegion(VibrancyType::ACTIVE_SOURCE_LIST_SELECTION,
                                     activeSourceListSelectionRegion);
 
+  UpdateInternalOpaqueRegion();
+
   if (changed) {
     SuspendAsyncCATransactions();
   }
@@ -2515,6 +2520,19 @@ mozilla::VibrancyManager& nsChildView::EnsureVibrancyManager() {
   return *mVibrancyManager;
 }
 
+void nsChildView::UpdateInternalOpaqueRegion() {
+  MOZ_RELEASE_ASSERT(NS_IsMainThread(), "This should only be called on the main thread.");
+  auto opaqueRegion = mOpaqueRegion.Lock();
+  bool widgetIsOpaque = GetTransparencyMode() == eTransparencyOpaque;
+  if (!widgetIsOpaque) {
+    opaqueRegion->SetEmpty();
+  } else if (VibrancyManager::SystemSupportsVibrancy()) {
+    opaqueRegion->Sub(mBounds, EnsureVibrancyManager().GetUnionOfVibrantRegions());
+  } else {
+    *opaqueRegion = mBounds;
+  }
+}
+
 nsChildView::SwipeInfo nsChildView::SendMayStartSwipe(
     const mozilla::PanGestureInput& aSwipeStartEvent) {
   nsCOMPtr<nsIWidget> kungFuDeathGrip(this);