Bug 1034240 - Trigger an update of the displayport margins on a new APZC if the content already has margins. r=botond

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2344,6 +2344,15 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
     mFrameMetrics = aLayerMetrics;
     mLastDispatchedPaintMetrics = aLayerMetrics;
     ShareCompositorFrameMetrics();
+
+    if (mFrameMetrics.GetDisplayPortMargins() != LayerMargin()) {
+      // A non-zero display port margin here indicates a displayport has
+      // been set by a previous APZC for the content at this guid. The
+      // scrollable rect may have changed since then, making the margins
+      // wrong, so we need to calculate a new display port.
+      APZC_LOG("%p detected non-empty margins which probably need updating\n", this);
+      needContentRepaint = true;
+    }
   } else {
     // If we're not taking the aLayerMetrics wholesale we still need to pull
     // in some things into our local mFrameMetrics because these things are