cc: Fix skewport math

cc/resources/picture_layer_tiling.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/resources/picture_layer_tiling.h"
 
 #include <algorithm>
 #include <cmath>
 #include <limits>
 #include <set>
@@ skipping 511 matching lines @@
     if (recycled_twin)
       recycled_twin->RemoveTileAt(it->first.first, it->first.second, nullptr);
   }
   tiles_.clear();
 }
 
 gfx::Rect PictureLayerTiling::ComputeSkewport(
     double current_frame_time_in_seconds,
     const gfx::Rect& visible_rect_in_content_space) const {
   gfx::Rect skewport = visible_rect_in_content_space;
+  if (skewport.IsEmpty())
+    return skewport;
+
   if (visible_rect_history_[1].frame_time_in_seconds == 0.0)
     return skewport;
 
   double time_delta = current_frame_time_in_seconds -
                       visible_rect_history_[1].frame_time_in_seconds;
   if (time_delta == 0.0)
     return skewport;
 
   double extrapolation_multiplier =
       skewport_target_time_in_seconds_ / time_delta;
@@ skipping 15 matching lines @@
   gfx::Rect max_skewport = skewport;
   max_skewport.Inset(-skewport_extrapolation_limit_in_content_pixels_,
                      -skewport_extrapolation_limit_in_content_pixels_);
 
   // Inset the skewport by the needed adjustment.
   skewport.Inset(extrapolation_multiplier * (new_x - old_x),
                  extrapolation_multiplier * (new_y - old_y),
                  extrapolation_multiplier * (old_right - new_right),
                  extrapolation_multiplier * (old_bottom - new_bottom));
 
-  // Clip the skewport to |max_skewport|.
+  // Ensure that visible rect is contained in the skewport.
+  skewport.Union(visible_rect_in_content_space);
+
+  // Clip the skewport to |max_skewport|. This needs to happen after the
+  // union in case intersecting would have left the empty rect.
   skewport.Intersect(max_skewport);
 
-  // Finally, ensure that visible rect is contained in the skewport.
-  skewport.Union(visible_rect_in_content_space);
   return skewport;
 }
 
 bool PictureLayerTiling::ComputeTilePriorityRects(
     const gfx::Rect& viewport_in_layer_space,
     float ideal_contents_scale,
     double current_frame_time_in_seconds,
     const Occlusion& occlusion_in_layer_space) {
   if (!NeedsUpdateForFrameAtTimeAndViewport(current_frame_time_in_seconds,
                                             viewport_in_layer_space)) {
@@ skipping 459 matching lines @@
       break;
   }
 
   gfx::Rect result(origin_x, origin_y, width, height);
   if (cache)
     cache->previous_result = result;
   return result;
 }
 
 }  // namespace cc