Bug 968222 - Update zoom scale in APZC upon orientation change. r=botond,kats

From f60ff14c5fdd1f7fef29ea5b2d7b4c8fa5710157 Mon Sep 17 00:00:00 2001
---
 gfx/layers/ipc/AsyncPanZoomController.cpp | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -1656,11 +1656,17 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
     // If we're not taking the aLayerMetrics wholesale we still need to pull
     // in some things into our local mFrameMetrics because these things are
     // determined by Gecko and our copy in mFrameMetrics may be stale.
+
+    if (mFrameMetrics.mCompositionBounds.width == aLayerMetrics.mCompositionBounds.width) {
+      float parentResolutionChange = aLayerMetrics.GetParentResolution().scale
+                                   / mFrameMetrics.GetParentResolution().scale;
+      mFrameMetrics.mZoom.scale *= parentResolutionChange;
+    } else {
+      // Take the new zoom as composition width got changed (i.e. due to orientation change)
+      mFrameMetrics.mZoom.scale = aLayerMetrics.mZoom.scale;
+    }
     mFrameMetrics.mScrollableRect = aLayerMetrics.mScrollableRect;
     mFrameMetrics.mCompositionBounds = aLayerMetrics.mCompositionBounds;
-    float parentResolutionChange = aLayerMetrics.GetParentResolution().scale
-                                 / mFrameMetrics.GetParentResolution().scale;
-    mFrameMetrics.mZoom.scale *= parentResolutionChange;
     mFrameMetrics.mResolution = aLayerMetrics.mResolution;
     mFrameMetrics.mCumulativeResolution = aLayerMetrics.mCumulativeResolution;
     mFrameMetrics.mHasScrollgrab = aLayerMetrics.mHasScrollgrab;