Dirty rects always contain full tiles with delegated rendering.

cc/layers/tiled_layer.cc

 // Copyright 2011 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/layers/tiled_layer.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "base/auto_reset.h"
@@ skipping 316 matching lines @@
                              const OcclusionTracker* occlusion,
                              bool* updated) {
   CreateUpdaterIfNeeded();
 
   bool ignore_occlusions = !occlusion;
   if (!HaveTexturesForTiles(left, top, right, bottom, ignore_occlusions)) {
     failed_update_ = true;
     return false;
   }
 
-  gfx::Rect paint_rect =
-      MarkTilesForUpdate(left, top, right, bottom, ignore_occlusions);
+  gfx::Rect update_rect;
+  gfx::Rect paint_rect;
+  MarkTilesForUpdate(
+    &update_rect, &paint_rect, left, top, right, bottom, ignore_occlusions);
 
   if (occlusion)
     occlusion->overdraw_metrics()->DidPaint(paint_rect);
 
   if (paint_rect.IsEmpty())
     return true;
 
   *updated = true;
   UpdateTileTextures(
-      paint_rect, left, top, right, bottom, queue, occlusion);
+      update_rect, paint_rect, left, top, right, bottom, queue, occlusion);
   return true;
 }
 
 void TiledLayer::MarkOcclusionsAndRequestTextures(
     int left,
     int top,
     int right,
     int bottom,
     const OcclusionTracker* occlusion) {
   // There is some difficult dependancies between occlusions, recording
@@ skipping 56 matching lines @@
       if (tile->occluded && !ignore_occlusions)
         continue;
 
       if (!tile->managed_resource()->can_acquire_backing_texture())
         return false;
     }
   }
   return true;
 }
 
-gfx::Rect TiledLayer::MarkTilesForUpdate(int left,
-                                         int top,
-                                         int right,
-                                         int bottom,
-                                         bool ignore_occlusions) {
-  gfx::Rect paint_rect;
+void TiledLayer::MarkTilesForUpdate(gfx::Rect* update_rect,
+                                    gfx::Rect* paint_rect,
+                                    int left,
+                                    int top,
+                                    int right,
+                                    int bottom,
+                                    bool ignore_occlusions) {
   for (int j = top; j <= bottom; ++j) {
     for (int i = left; i <= right; ++i) {
       UpdatableTile* tile = TileAt(i, j);
       DCHECK(tile);  // Did SetTexturePriorites get skipped?
                      // TODO(enne): This should not ever be null.
       if (!tile)
         continue;
       if (tile->occluded && !ignore_occlusions)
         continue;
+
+      // Prepare update rect from original dirty rects.
+      update_rect->Union(tile->dirty_rect);
+
       // TODO(reveman): Decide if partial update should be allowed based on cost
       // of update. https://bugs.webkit.org/show_bug.cgi?id=77376
       if (tile->is_dirty() &&
           !layer_tree_host()->AlwaysUsePartialTextureUpdates()) {
         // If we get a partial update, we use the same texture, otherwise return
         // the current texture backing, so we don't update visible textures
         // non-atomically.  If the current backing is in-use, it won't be
         // deleted until after the commit as the texture manager will not allow
         // deletion or recycling of in-use textures.
         if (TileOnlyNeedsPartialUpdate(tile) &&
             layer_tree_host()->RequestPartialTextureUpdate()) {
           tile->partial_update = true;
         } else {
           tile->dirty_rect = tiler_->TileRect(tile);
           tile->managed_resource()->ReturnBackingTexture();
         }
       }
 
-      paint_rect.Union(tile->dirty_rect);
+      paint_rect->Union(tile->dirty_rect);
       tile->MarkForUpdate();
     }
   }
-  return paint_rect;
 }
 
-void TiledLayer::UpdateTileTextures(gfx::Rect paint_rect,
+void TiledLayer::UpdateTileTextures(gfx::Rect update_rect,
+                                    gfx::Rect paint_rect,
                                     int left,
                                     int top,
                                     int right,
                                     int bottom,
                                     ResourceUpdateQueue* queue,
                                     const OcclusionTracker* occlusion) {
   // The update_rect should be in layer space. So we have to convert the
   // paint_rect from content space to layer space.
   float width_scale =
       paint_properties().bounds.width() /
       static_cast<float>(content_bounds().width());
   float height_scale =
       paint_properties().bounds.height() /
       static_cast<float>(content_bounds().height());
-  update_rect_ = gfx::ScaleRect(paint_rect, width_scale, height_scale);
+  update_rect_ = gfx::ScaleRect(update_rect, width_scale, height_scale);
 
   // Calling PrepareToUpdate() calls into WebKit to paint, which may have the
   // side effect of disabling compositing, which causes our reference to the
   // texture updater to be deleted.  However, we can't free the memory backing
   // the SkCanvas until the paint finishes, so we grab a local reference here to
   // hold the updater alive until the paint completes.
   scoped_refptr<LayerUpdater> protector(Updater());
   gfx::Rect painted_opaque_rect;
   Updater()->PrepareToUpdate(paint_rect,
                              tiler_->tile_size(),
@@ skipping 413 matching lines @@
   gfx::Rect prepaint_rect = visible_content_rect();
   prepaint_rect.Inset(-tiler_->tile_size().width() * kPrepaintColumns,
                       -tiler_->tile_size().height() * kPrepaintRows);
   gfx::Rect content_rect(content_bounds());
   prepaint_rect.Intersect(content_rect);
 
   return prepaint_rect;
 }
 
 }  // namespace cc