Handle tile prioritization correctly when global visible rect is empty.

android_webview/browser/browser_view_renderer.cc

 // Copyright 2014 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 "android_webview/browser/browser_view_renderer.h"
 
 #include "android_webview/browser/browser_view_renderer_client.h"
 #include "android_webview/browser/shared_renderer_state.h"
 #include "android_webview/common/aw_switches.h"
 #include "android_webview/public/browser/draw_gl.h"
@@ skipping 281 matching lines @@
   gfx::Rect clip = viewport;
   gfx::Transform transform_for_tile_priority =
       parent_draw_constraints_.transform;
 
   // If the WebView is on a layer, WebView does not know what transform is
   // applied onto the layer so global visible rect does not make sense here.
   // In this case, just use the surface rect for tiling.
   gfx::Rect viewport_rect_for_tile_priority;
   if (parent_draw_constraints_.is_layer)
     viewport_rect_for_tile_priority = parent_draw_constraints_.surface_rect;
-  else
+  else if (!last_on_draw_global_visible_rect_.IsEmpty())
     viewport_rect_for_tile_priority = last_on_draw_global_visible_rect_;
+  else {
+    // An empty viewport rect for tile priority will make cc fallback to use
+    // the WebView size as viewport rect for tile priority, which is not what
+    // we want. Instead, use the smallest non-empty rect for it.
+    viewport_rect_for_tile_priority = gfx::Rect(0, 0, 1, 1);
+  }
 
   scoped_ptr<cc::CompositorFrame> frame =
       compositor_->DemandDrawHw(surface_size,
                                 gfx::Transform(),
                                 viewport,
                                 clip,
                                 viewport_rect_for_tile_priority,
                                 transform_for_tile_priority);
   if (!frame.get())
     return false;
@@ skipping 501 matching lines @@
     base::StringAppendF(&str,
                         "surface width height: [%d %d] ",
                         draw_info->width,
                         draw_info->height);
     base::StringAppendF(&str, "is_layer: %d ", draw_info->is_layer);
   }
   return str;
 }
 
 }  // namespace android_webview