Have Chrome draw top window border when using custom titlebar.

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

Index: chrome/browser/ui/views/frame/browser_desktop_window_tree_host_win.cc
diff --git a/chrome/browser/ui/views/frame/browser_desktop_window_tree_host_win.cc b/chrome/browser/ui/views/frame/browser_desktop_window_tree_host_win.cc
index 406f558591156c6dcc7fb37d47dec0a7aa77d122..c37a63b2bb4ae0a9b66d30464e8dab23e3678dfb 100644
--- a/chrome/browser/ui/views/frame/browser_desktop_window_tree_host_win.cc
+++ b/chrome/browser/ui/views/frame/browser_desktop_window_tree_host_win.cc
@@ -173,10 +173,10 @@ void BrowserDesktopWindowTreeHostWin::PostHandleMSG(UINT message,
       // unless necessary to avoid flicker. This may be invoked during creation
       // on XP and before the non_client_view has been created.
       WINDOWPOS* window_pos = reinterpret_cast<WINDOWPOS*>(l_param);
-      if (window_pos->flags & SWP_SHOWWINDOW &&
-          GetWidget()->non_client_view()) {
-        GetWidget()->non_client_view()->Layout();
-        GetWidget()->non_client_view()->SchedulePaint();
+      auto* non_client_view = GetWidget()->non_client_view();
+      if (window_pos->flags & SWP_SHOWWINDOW && non_client_view) {
+        non_client_view->Layout();
+        non_client_view->SchedulePaint();
       }
       break;
     }
@@ -199,6 +199,13 @@ void BrowserDesktopWindowTreeHostWin::PostHandleMSG(UINT message,
       }
       break;
     }
+    case WM_DWMCOLORIZATIONCOLORCHANGED: {
+      auto* non_client_view = GetWidget()->non_client_view();
+      // Activation border may have changed color.

[Peter Kasting, 2016/10/06 08:02:47]

Grammar Nazi Nit: Add "The" in front

+      if (non_client_view)
+        non_client_view->SchedulePaint();
+      break;
+    }
   }
 }
 
@@ -250,20 +257,6 @@ void BrowserDesktopWindowTreeHostWin::FrameTypeChanged() {
 // BrowserDesktopWindowTreeHostWin, private:
 
 void BrowserDesktopWindowTreeHostWin::UpdateDWMFrame() {
-  // With a custom titlebar we want the margins to always be 2 pixels, because
-  // that gives us the 1 pixel accent color border around the window (a 1 pixel
-  // margin is not sufficient, it will draw a messed-up-looking border instead).
-  // The other pixel ends up being 1-pixel-thick native titlebar (including
-  // caption buttons) but since we draw over that pixel in
-  // GlassBrowserFrameView::PaintTitlebar() it will be invisible and won't get
-  // mouse events.
-  if (browser_frame_->CustomDrawSystemTitlebar() && ShouldUseNativeFrame() &&
-      !GetWidget()->IsFullscreen()) {
-    MARGINS margins = {2, 2, 2, 2};
-    DwmExtendFrameIntoClientArea(GetHWND(), &margins);
-    return;
-  }
-
   // For "normal" windows on Aero, we always need to reset the glass area
   // correctly, even if we're not currently showing the native frame (e.g.
   // because a theme is showing), so we explicitly check for that case rather
@@ -299,9 +292,10 @@ BrowserDesktopWindowTreeHostWin::GetClientEdgeThicknesses() const {
 }
 
 MARGINS BrowserDesktopWindowTreeHostWin::GetDWMFrameMargins() const {
-  // If we're using the opaque frame or we're fullscreen we don't extend the
-  // glass in at all because it won't be visible.
-  if (!ShouldUseNativeFrame() || GetWidget()->IsFullscreen())
+  // If we're using the opaque frame or custom titlebar or we're fullscreen we
+  // don't extend the glass in at all because it won't be visible.

[Peter Kasting, 2016/10/06 08:02:47]

Nit: Clearer (to me):

Don't extend the DWM frame in unless we're letting the DWM draw a non-maximized,
non-fullscreen window.  Outside that scenario, no DWM-drawn area will be
visible.

[Bret, 2016/10/06 20:48:10]

Not quite accurate because DWM still draws non-client area, and it doesn't
explain why the custom titlebar part is important unless you're familiar with
how it works. I think the comment is okay as is.

[Peter Kasting, 2016/10/11 20:01:38]

What about:

Don't extend the DWM frame in unless we're letting the DWM draw a non-maximized,
non-fullscreen window.  Outside that scenario, the DWM will not be drawing in
the window client area.

This corrects the inaccuracy you mention.  The reason I'm suggesting a change is
because I'd like to spend less time restating the code and more time on "why". 
It's subtle, though.

[Bret, 2016/10/11 20:48:19]

Okay, I agree about not restating the code. Maybe this doesn't need a comment at
all? Or if you think it does, maybe just "Don't extend the DWM frame in when it
won't be visible." I still think "won't be visible" is more accurate because DWM
still draws things when even when we draw over it (you can tell if Chrome dies
somehow and only the Windows-drawn area is visible).

[Peter Kasting, 2016/10/11 20:56:51]

I like your simple comment suggestion.

+  if (!ShouldUseNativeFrame() || GetWidget()->IsFullscreen() ||
+      browser_frame_->CustomDrawSystemTitlebar())
     return MARGINS{0};
 
   // The glass should extend to the bottom of the tabstrip.