Bug 1933423. Only create nsDisplayFixedPosition if painting to the window. r=botond

The only thing that useFixedPosition controls is whether we create a nsDisplayFixedPosition item. The only things that nsDisplayFixedPosition does different are things that only affect painting to the window (CreateWebRenderCommands, UpdateScrollData, ShouldGetFixedAnimationId).

I don't think this causes any behaviour difference, but we only need a nsDisplayFixedPosition item if we are painting to the window.

Differential Revision: https://phabricator.services.mozilla.com/D230243

Author: tnikkel

layout/generic/nsIFrame.cpp

@@ -3335,6 +3335,7 @@ void nsIFrame::BuildDisplayListForStackingContext(
 
   bool useFixedPosition =
       disp->mPosition == StylePositionProperty::Fixed &&
+      aBuilder->IsPaintingToWindow() &&
       (DisplayPortUtils::IsFixedPosFrameInDisplayPort(this) ||
        BuilderHasScrolledClip(aBuilder));