--- a/gfx/layers/Layers.cpp
+++ b/gfx/layers/Layers.cpp
@@ -1845,16 +1845,19 @@ Layer::PrintInfo(std::stringstream& aStr
 
   layers::PrintInfo(aStream, AsHostLayer());
 
   if (mClipRect) {
     AppendToString(aStream, *mClipRect, " [clip=", "]");
   }
   if (mSimpleAttrs.ScrolledClip()) {
     AppendToString(aStream, mSimpleAttrs.ScrolledClip()->GetClipRect(), " [scrolled-clip=", "]");
+    if (const Maybe<size_t>& ix = mSimpleAttrs.ScrolledClip()->GetMaskLayerIndex()) {
+      AppendToString(aStream, ix.value(), " [scrolled-mask=", "]");
+    }
   }
   if (1.0 != mSimpleAttrs.PostXScale() || 1.0 != mSimpleAttrs.PostYScale()) {
     aStream << nsPrintfCString(" [postScale=%g, %g]", mSimpleAttrs.PostXScale(), mSimpleAttrs.PostYScale()).get();
   }
   if (!GetBaseTransform().IsIdentity()) {
     AppendToString(aStream, GetBaseTransform(), " [transform=", "]");
   }
   if (!GetEffectiveTransform().IsIdentity()) {

--- a/gfx/layers/LayersLogging.cpp
+++ b/gfx/layers/LayersLogging.cpp
@@ -165,16 +165,19 @@ AppendToString(std::stringstream& aStrea
   AppendToString(aStream, m.GetMetrics(), "{ [metrics=");
   AppendToString(aStream, m.GetBackgroundColor(), "] [color=");
   if (m.GetScrollParentId() != FrameMetrics::NULL_SCROLL_ID) {
     AppendToString(aStream, m.GetScrollParentId(), "] [scrollParent=");
   }
   if (m.HasScrollClip()) {
     AppendToString(aStream, m.ScrollClip().GetClipRect(), "] [clip=");
   }
+  if (m.HasMaskLayer()) {
+    AppendToString(aStream, m.ScrollClip().GetMaskLayerIndex().value(), "] [mask=");
+  }
   aStream << "] }" << sfx;
 }
 
 void
 AppendToString(std::stringstream& aStream, const FrameMetrics& m,
                const char* pfx, const char* sfx, bool detailed)
 {
   aStream << pfx;