Bug 1923790. Skip checking for combine 3d in GetResultingTransformMatrix when we already know there is no combine 3d. r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D225171

layout/generic/nsIFrame.cpp

@@ -3269,6 +3269,9 @@ void nsIFrame::BuildDisplayListForStackingContext(
         if (!hasPerspective) {
           flags &= ~nsDisplayTransform::INCLUDE_PERSPECTIVE;
         }
+        if (!combines3DTransformWithAncestors) {
+          flags &= ~nsDisplayTransform::INCLUDE_PRESERVE3D_ANCESTORS;
+        }
         auto transform = nsDisplayTransform::GetResultingTransformMatrix(
             this, nsPoint(), appPerDev, flags);
         nsRect untransformedDirtyRect;