[ash-md] Improves smoothness with many windows in overview

ash/common/wm/overview/window_grid.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 "ash/common/wm/overview/window_grid.h"
 
 #include <algorithm>
 #include <functional>
 #include <set>
 #include <utility>
@@ skipping 79 matching lines @@
 
 // Conceptually the window overview is a table or grid of cells having this
 // fixed aspect ratio. The number of columns is determined by maximizing the
 // area of them based on the number of window_list.
 const float kCardAspectRatio = 4.0f / 3.0f;
 
 // The minimum number of cards along the major axis (i.e. horizontally on a
 // landscape orientation).
 const int kMinCardsMajor = 3;
 
+// Hiding windows headers can be resource intensive. Only hide the headers when

[tdanderson, 2016/07/18 16:26:58]

nit: windows->window

[varkha, 2016/07/19 00:29:07]

Done.

+// the number of windows in this grid is less or equal than this number.
+const int kMaxWindowsCountToHideHeader = 10;
+
 const int kOverviewSelectorTransitionMilliseconds = 250;
 
 // The color and opacity of the screen shield in overview.
 const SkColor kShieldColor = SkColorSetARGB(178, 0, 0, 0);
 
 // The color and opacity of the overview selector.
 const SkColor kWindowSelectionColor = SkColorSetARGB(128, 0, 0, 0);
 const SkColor kWindowSelectionColorMD = SkColorSetARGB(51, 255, 255, 255);
 const SkColor kWindowSelectionBorderColor = SkColorSetARGB(38, 255, 255, 255);
 const SkColor kWindowSelectionBorderColorMD = SkColorSetARGB(76, 255, 255, 255);
@@ skipping 329 matching lines @@
 void WindowGrid::PrepareForOverview() {
   if (ash::MaterialDesignController::IsOverviewMaterial())
     InitShieldWidget();
   for (auto iter = window_list_.begin(); iter != window_list_.end(); ++iter)
     (*iter)->PrepareForOverview();
 }
 
 void WindowGrid::PositionWindowsMD(bool animate) {
   if (window_list_.empty())
     return;
+  const bool hide_header = window_list_.size() <= kMaxWindowsCountToHideHeader;
+  for (auto* window : window_list_)
+    window->set_hide_header(hide_header);
   gfx::Rect total_bounds =
       root_window_->ConvertRectToScreen(wm::GetDisplayWorkAreaBoundsInParent(
           root_window_->GetChildByShellWindowId(
               kShellWindowId_DefaultContainer)));
   // Windows occupy vertically centered area with additional vertical insets.
   int horizontal_inset =
       gfx::ToFlooredInt(std::min(kOverviewInsetRatio * total_bounds.width(),
                                  kOverviewInsetRatio * total_bounds.height()));
   int vertical_inset =
       horizontal_inset +
@@ skipping 534 matching lines @@
         *min_right = left;
       if (*max_right < left)
         *max_right = left;
     }
     *max_bottom = top + height;
   }
   return windows_fit;
 }
 
 }  // namespace ash