Bug 1687928. Move call to MaybeCreateDisplayPortInFirstScrollFrameEncountered before DecideScrollableLayer in PaintFrame. r=botond

Because DecideScrollableLayer uses the existence of a displayport in it's decision.

In practice though we only paint from process root documents and the root scroll frame will have a displayport (almost?) always. So it's a small correctness improvement that is likely to have little effect.

Differential Revision: https://phabricator.services.mozilla.com/D102588

layout/base/nsLayoutUtils.cpp

@@ -3169,6 +3169,12 @@ nsresult nsLayoutUtils::PaintFrame(gfxContext* aRenderingContext,
     }
   }
 
+  builder->ClearHaveScrollableDisplayPort();
+  if (builder->IsPaintingToWindow()) {
+    DisplayPortUtils::MaybeCreateDisplayPortInFirstScrollFrameEncountered(
+        aFrame, builder);
+  }
+
   nsIFrame* rootScrollFrame = presShell->GetRootScrollFrame();
   if (rootScrollFrame && !aFrame->GetParent()) {
     nsIScrollableFrame* rootScrollableFrame =
@@ -3226,12 +3232,6 @@ nsresult nsLayoutUtils::PaintFrame(gfxContext* aRenderingContext,
     }
   }
 
-  builder->ClearHaveScrollableDisplayPort();
-  if (builder->IsPaintingToWindow()) {
-    DisplayPortUtils::MaybeCreateDisplayPortInFirstScrollFrameEncountered(
-        aFrame, builder);
-  }
-
   nsRect visibleRect = visibleRegion.GetBounds();
   PartialUpdateResult updateState = PartialUpdateResult::Failed;