--- a/gfx/layers/wr/WebRenderCommandBuilder.cpp
+++ b/gfx/layers/wr/WebRenderCommandBuilder.cpp
@@ -1603,19 +1603,19 @@ WebRenderCommandBuilder::GenerateFallbac
   // has some sort of scaling applied.
   LayerIntSize paintSize = RoundedToInt(LayerSize(bounds.Width() * scale.width, bounds.Height() * scale.height));
   if (paintSize.width == 0 || paintSize.height == 0) {
     return nullptr;
   }
 
   // Some display item may draw exceed the paintSize, we need prepare a larger
   // draw target to contain the result.
-  auto scaledBounds = bounds * LayoutDeviceToLayerScale(1);
-  scaledBounds.Scale(scale.width, scale.height);
-  LayerIntSize dtSize = RoundedToInt(scaledBounds).Size();
+  auto scaledPaintSize = bounds.Size() * LayoutDeviceToLayerScale(1);
+  scaledPaintSize.Scale(scale.width, scale.height);
+  LayerIntSize dtSize = RoundedToInt(scaledPaintSize);
 
   bool needPaint = true;
   LayoutDeviceIntPoint offset = RoundedToInt(bounds.TopLeft());
   aImageRect = LayoutDeviceRect(offset, LayoutDeviceSize(RoundedToInt(bounds).Size()));
   LayerRect paintRect = LayerRect(LayerPoint(0, 0), LayerSize(paintSize));
   nsDisplayItemGeometry* geometry = fallbackData->GetGeometry();
 
   // nsDisplayFilter is rendered via BasicLayerManager which means the invalidate