Where an ancestor has overflow style, a fixed child needs to be composited.

Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

Index: Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
diff --git a/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp b/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
index 029fa2bff9e0267b019e263cc92f8a2f9451e8c2..b5fe90799d4c8e4e35eb24993c22140749348424 100644
--- a/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
+++ b/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
@@ -223,6 +223,9 @@ void CompositingRequirementsUpdater::updateRecursive(DeprecatedPaintLayer* ances
     if (currentRecursionData.m_compositingAncestor && currentRecursionData.m_compositingAncestor->layoutObject()->isVideo())
         directReasons |= CompositingReasonVideoOverlay;
 
+    if (currentRecursionData.m_hasCompositedScrollingAncestor && layer->layoutObject()->styleRef().hasViewportConstrainedPosition())
+        directReasons |= CompositingReasonPositionFixed;
+
     if (compositor->canBeComposited(layer)) {
         reasonsToComposite |= directReasons;