Have Chrome draw top window border when using custom titlebar.

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

Index: chrome/browser/ui/views/frame/glass_browser_frame_view.cc
diff --git a/chrome/browser/ui/views/frame/glass_browser_frame_view.cc b/chrome/browser/ui/views/frame/glass_browser_frame_view.cc
index 98d1063b2ea1f5353d5ccd642c28927d41fbee11..58ec6a93a39c76389ac1ce80472370aad594aa41 100644
--- a/chrome/browser/ui/views/frame/glass_browser_frame_view.cc
+++ b/chrome/browser/ui/views/frame/glass_browser_frame_view.cc
@@ -498,6 +498,11 @@ Windows10CaptionButton* GlassBrowserFrameView::CreateCaptionButton(
 }
 
 void GlassBrowserFrameView::PaintTitlebar(gfx::Canvas* canvas) const {
+  // Normally the inactive accent border is semi-transparent, but we want to
+  // make it opaque so that DWM won't need to blend with windows behind
+  // Chrome. This color is the same as Edge uses and closely matches the
+  // appearance of the normal inactive accent border.
+  constexpr SkColor inactive_border_color = 0xFFA2A2A2;
   SkColor frame_color = 0xFFCCCCCC;
   gfx::Rect tabstrip_bounds = GetBoundsForTabStrip(browser_view()->tabstrip());

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

Nit: Let's move these constants down to just above the code that uses them.  We
can then combine the comment added here with the one below.  See next comment
for proposed wording.

 
@@ -509,14 +514,24 @@ void GlassBrowserFrameView::PaintTitlebar(gfx::Canvas* canvas) const {
   // BrowserDesktopWindowTreeHostWin::UpdateDWMFrame()).
   const int y = IsMaximized() ? FrameTopBorderThicknessPx(false) : 1;
   SkPaint paint;
+
+  // Draw the top of the accent border. DWM does this for the other sides of
+  // the window, and Chrome uses an inset to avoid drawing over top of them.
+  // However the top of the window must have an inset of 0 or else DWM draws a
+  // full set of caption buttons outside the window.
+  //
+  // Chrome should draw this itself instead of using transparency so that
+  // alpha blending can be disabled on the window, which saves power when
+  // compositing.

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

Nit: This comment partially restates info from the comment above.  If we move
the constant down anyway and combine the comments, here's a suggestion I hope
gives much more background for future puzzled spelunkers:

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 (see
BrowserDesktopWindowTreeHostWin::GetClientAreaInsets()).  However, along the top
window edge, we have to have zero nonclient area, or the DWM will draw a full
set of caption buttons outside our client area.  [pk note: I think saying
"outside our client area" is more correct than "outside the window", but not
sure.  Also, some of this info probably belongs in
BrowserDesktopWindowTreeHostWin::GetClientAreaInsets() in addition to/instead of
here.]

We could ask the DWM to draw the top accent border in the client area (by
ensuring the top margin is nonzero in
BrowserDesktopWindowTreeHostWin::UpdateDWMFrame()), but this requires that we
leave part of the client surface transparent and let the DWM draw the contents
underneath and blend.  This costs power (~10% extra draw playing a maximized
1080p60 video).  If we draw this ourselves, we can make the client surface fully
opaque and avoid this cost.

However, this requires that the accent border be fully opaque.  Active window
accent borders are opaque, and we can get the correct color from the theme
provider.  Inactive window accent borders are natively ARGB 0x80565656.  There's
no single opaque color that will match that against all backgrounds, and we
don't know what's behind us anyway.  So we copy Edge (which also has an opaque
top border) and assume most users will have relatively bright backgrounds; they
use #A2A2A2 for the inactive top border, which matches what the native border
would look like atop #EFEFEF.

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

I like Peter's suggestion in general. I made some edits:

"
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 [add a similar sentence to
BrowserDesktopWindowTreeHostWin::GetClientAreaInsets too]. 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 when the window isn't visible [I would avoid specific data here]. 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.
"

In editing that comment I realized that Edge uses #090909 when it's dark-themed.
Add a TODO somewhere since we don't have dark theme support yet and this is easy
to miss when we add it.

+  paint.setColor(
+      ShouldPaintAsActive()
+          ? GetThemeProvider()->GetColor(ThemeProperties::COLOR_ACCENT_BORDER)
+          : inactive_border_color);
+  canvas->DrawRect(gfx::RectF(0, 0, width() * scale, y), paint);

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

Nit: Blank line below this, since the comment above doesn't apply to the
subsequent code.

   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 {