--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -9660,16 +9660,20 @@ nsDisplayMask::CreateWebRenderCommands(m
   if (mask) {
     auto layoutBounds = wr::ToRoundedLayoutRect(bounds);
     wr::WrClipId clipId = aBuilder.DefineClip(Nothing(), Nothing(),
         layoutBounds, nullptr, mask.ptr());
 
     // Create a new stacking context to attach the mask to, ensuring the mask is
     // applied to the aggregate, and not the individual elements.
 
+    // The stacking context shouldn't have any offset.
+    layoutBounds.origin.x = 0;
+    layoutBounds.origin.y = 0;
+
     aBuilder.PushStackingContext(/*aBounds: */ layoutBounds,
                                  /*aClipNodeId: */ &clipId,
                                  /*aAnimation: */ nullptr,
                                  /*aOpacity: */ nullptr,
                                  /*aTransform: */ nullptr,
                                  /*aTransformStyle: */ wr::TransformStyle::Flat,
                                  /*aPerspective: */ nullptr,
                                  /*aMixBlendMode: */ wr::MixBlendMode::Normal,