Have Chrome draw top window border when using custom titlebar.

chrome/browser/ui/views/frame/glass_browser_frame_view.cc

 // Copyright (c) 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 "chrome/browser/ui/views/frame/glass_browser_frame_view.h"
 
 #include <dwmapi.h>
 #include <utility>
 
 #include "base/win/windows_version.h"
@@ skipping 491 matching lines @@
   gfx::Rect tabstrip_bounds = GetBoundsForTabStrip(browser_view()->tabstrip());
 
   gfx::ScopedCanvas scoped_canvas(canvas);
   float scale = canvas->UndoDeviceScaleFactor();
   // This is the pixel-accurate version of WindowTopY(). Scaling the DIP values
   // here compounds precision error, which exposes the native Windows caption
   // buttons we need to draw over. (see the comment in
   // BrowserDesktopWindowTreeHostWin::UpdateDWMFrame()).
   const int y = IsMaximized() ? FrameTopBorderThicknessPx(false) : 1;
   SkPaint paint;
+
+  // Draw the top of the accent border.
+  //
+  // We let the DWM do this for the other sides of the window by insetting the
+  // client area to leave nonclient area available. However, along the top
+  // window edge, we have to have zero nonclient area or the DWM will draw a
+  // full native titlebar outside our client area. See
+  // BrowserDesktopWindowTreeHostWin::GetClientAreaInsets().
+  //
+  // We could ask the DWM to draw the top accent border in the client area (by
+  // calling DwmExtendFrameIntoClientArea in
+  // BrowserDesktopWindowTreeHostWin::UpdateDWMFrame()), but this requires
+  // that we leave part of the client surface transparent. Forcing DWM to
+  // blend with us costs power even for the parts of the window that are

[Bret, 2016/10/07 18:21:56]

nit: "Forcing DWM to blend with us costs power even when it has no effect."?

+  // opaque. If we draw this ourselves, we can make the client surface fully
+  // opaque and avoid this cost.
+  //
+  // So the accent border also has to be opaque, but native inactive borders
+  // are #565656 with 80% alpha. We copy Edge (which also custom-draws its top
+  // border) and use #A2A2A2 instead.
+  // TODO(jbauman): Match Edge's #090909 when using the dark theme.
+  constexpr SkColor inactive_border_color = 0xFFA2A2A2;
+  paint.setColor(
+      ShouldPaintAsActive()
+          ? GetThemeProvider()->GetColor(ThemeProperties::COLOR_ACCENT_BORDER)
+          : inactive_border_color);
+  canvas->DrawRect(gfx::RectF(0, 0, width() * scale, y), paint);
+
   paint.setColor(frame_color);
   canvas->DrawRect(
       gfx::RectF(0, y, width() * scale, tabstrip_bounds.bottom() * scale - y),
       paint);
-
-  // The 1 pixel line at the top is drawn by Windows when we leave that section
-  // of the window blank because we have called DwmExtendFrameIntoClientArea()
-  // inside BrowserDesktopWindowTreeHostWin::UpdateDWMFrame().
 }
 
 void GlassBrowserFrameView::PaintToolbarBackground(gfx::Canvas* canvas) const {
   // TODO(estade): can this be shared with OpaqueBrowserFrameView?
   gfx::Rect toolbar_bounds(browser_view()->GetToolbarBounds());
   if (toolbar_bounds.IsEmpty())
     return;
   gfx::Point toolbar_origin(toolbar_bounds.origin());
   ConvertPointToTarget(browser_view(), this, &toolbar_origin);
   toolbar_bounds.set_origin(toolbar_origin);
@@ skipping 245 matching lines @@
   static bool initialized = false;
   if (!initialized) {
     for (int i = 0; i < kThrobberIconCount; ++i) {
       throbber_icons_[i] =
           ui::LoadThemeIconFromResourcesDataDLL(IDI_THROBBER_01 + i);
       DCHECK(throbber_icons_[i]);
     }
     initialized = true;
   }
 }