--- a/widget/cocoa/nsChildView.h
+++ b/widget/cocoa/nsChildView.h
@@ -630,16 +630,17 @@ protected:
   // Compositor thread only
   mozilla::UniquePtr<mozilla::widget::RectTextureImage> mResizerImage;
   mozilla::UniquePtr<mozilla::widget::RectTextureImage> mCornerMaskImage;
   mozilla::UniquePtr<mozilla::widget::RectTextureImage> mTitlebarImage;
   mozilla::UniquePtr<mozilla::widget::RectTextureImage> mBasicCompositorImage;
 
   // Main thread + webrender only
   mozilla::Maybe<mozilla::wr::ImageKey> mTitlebarImageKey;
+  mozilla::gfx::IntSize mTitlebarImageSize;
 
   // The area of mTitlebarCGContext that has changed and needs to be
   // uploaded to to mTitlebarImage. Main thread only.
   nsIntRegion           mDirtyTitlebarRegion;
 
   mozilla::ViewRegion   mNonDraggableRegion;
 
   // Cached value of [mView backingScaleFactor], to avoid sending two obj-c

--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -2091,19 +2091,27 @@ nsChildView::AddWindowOverlayWebRenderCo
                       CGBitmapContextGetHeight(mTitlebarCGContext));
     size_t stride = CGBitmapContextGetBytesPerRow(mTitlebarCGContext);
     size_t titlebarCGContextDataLength = stride * size.height;
     gfx::SurfaceFormat format = gfx::SurfaceFormat::B8G8R8A8;
     wr::ByteBuffer buffer(
       titlebarCGContextDataLength,
       static_cast<uint8_t *>(CGBitmapContextGetData(mTitlebarCGContext)));
 
+    if (mTitlebarImageKey &&
+        mTitlebarImageSize != size) {
+      // Delete wr::ImageKey. wr::ImageKey does not support size change.
+      CleanupWebRenderWindowOverlay(aWrBridge);
+      MOZ_ASSERT(mTitlebarImageKey.isNothing());
+    }
+
     if (!mTitlebarImageKey) {
       mTitlebarImageKey = Some(aWrBridge->GetNextImageKey());
       aWrBridge->SendAddImage(*mTitlebarImageKey, size, stride, format, buffer);
+      mTitlebarImageSize = size;
       updatedTitlebarRegion.SetEmpty();
     }
 
     if (!updatedTitlebarRegion.IsEmpty()) {
       aWrBridge->SendUpdateImage(*mTitlebarImageKey, size, format, buffer);
     }
 
     WrRect rect = wr::ToWrRect(mTitlebarRect);