Add app.window.alphaEnabled() and onAlphaEnabledChanged.

apps/ui/views/native_app_window_views.cc

Index: apps/ui/views/native_app_window_views.cc
diff --git a/apps/ui/views/native_app_window_views.cc b/apps/ui/views/native_app_window_views.cc
index 62b941d9e58db4628f080934c6bf49b1660b76a8..5fdbc4e839d66be40fdb860803477cc4d0ef58bb 100644
--- a/apps/ui/views/native_app_window_views.cc
+++ b/apps/ui/views/native_app_window_views.cc
@@ -27,14 +27,12 @@ NativeAppWindowViews::NativeAppWindowViews()
       web_view_(NULL),
       widget_(NULL),
       frameless_(false),
-      transparent_background_(false),
       resizable_(false) {}
 
 void NativeAppWindowViews::Init(AppWindow* app_window,
                                 const AppWindow::CreateParams& create_params) {
   app_window_ = app_window;
   frameless_ = create_params.frame == AppWindow::FRAME_NONE;
-  transparent_background_ = create_params.transparent_background;
   resizable_ = create_params.resizable;
   size_constraints_.set_minimum_size(
       create_params.GetContentMinimumSize(gfx::Insets()));
@@ -260,7 +258,8 @@ void NativeAppWindowViews::OnWidgetActivationChanged(views::Widget* widget,
 
 void NativeAppWindowViews::RenderViewCreated(
     content::RenderViewHost* render_view_host) {
-  if (transparent_background_) {
+  if (app_window_->RequestedTransparentBackground() &&
+      CanHaveTransparentBackground()) {
     content::RenderWidgetHostView* view = render_view_host->GetView();
     DCHECK(view);
     view->SetBackgroundOpaque(false);
@@ -398,4 +397,12 @@ void NativeAppWindowViews::SetContentSizeConstraints(
   size_constraints_.set_maximum_size(max_size);
 }
 
+bool NativeAppWindowViews::CanHaveTransparentBackground() const {
+  return true;

[Wez, 2014/07/09 22:03:30]

So by default we're assuming that Views can always support transparency, unless
the platform-specific impl overrides this?

[jackhou1, 2014/07/10 03:04:18]

It looks like we use ChromeNativeAppWindowViews on CrOS and Linux. This logic
should probably live in the respective *TreeHost.

I'm not really sure what would be the idiomatic way to do this in views. Maybe
something like:
NativeAppWindowViews::CanHaveAlphaEnabled()
  calls views::Widget::CanHaveAlphaEnabled()
    calls WindowTreeHost::CanHaveAlphaEnabled()
      implemented by AppWindowDesktopWindowTreeHostWin?

[Wez, 2014/07/11 17:14:10]

Looks plausible, but I'm no expect on views/Aura.

[jackhou1, 2014/07/14 03:08:12]

benwells, any advice on this?

+}
+
+void NativeAppWindowViews::CanHaveTransparentBackgroundChanged() {
+  app_window_->OnNativeWindowChanged();

[Wez, 2014/07/09 22:03:30]

Does this fix the Classic->Aero transitions?

[jackhou1, 2014/07/10 03:04:18]

This just updates alphaEnabled in the javascript context.

[Wez, 2014/07/11 17:14:10]

Acknowledged.

+}
+
 }  // namespace apps