Gtk3: Render a GtkHeaderBar as the background of the tab strip

ui/native_theme/native_theme.h

 // 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.
 
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_H_
 
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "build/build_config.h"
@@ skipping 27 matching lines @@
 // ExtraParams union.  Each part that requires more information has its own
 // field in the union.
 //
 // NativeTheme also supports getting the default size of a given part with
 // the GetPartSize() method.
 class NATIVE_THEME_EXPORT NativeTheme {
  public:
   // The part to be painted / sized.
   enum Part {
     kCheckbox,
+    kFrameTopArea,
     kInnerSpinButton,
     kMenuList,
     kMenuPopupBackground,
 #if defined(OS_WIN)
     kMenuCheck,
     kMenuCheckBackground,
     kMenuPopupArrow,
     kMenuPopupGutter,
     kMenuPopupSeparator,
 #endif
@@ skipping 44 matching lines @@
   struct ButtonExtraParams {
     bool checked;
     bool indeterminate;  // Whether the button state is indeterminate.
     bool is_default;  // Whether the button is default button.
     bool is_focused;
     bool has_border;
     int classic_state;  // Used on Windows when uxtheme is not available.
     SkColor background_color;
   };
 
+  struct FrameTopAreaExtraParams {
+    bool is_active;
+    SkColor default_background_color;  // The theme can ignore this color.

[Tom (Use chromium acct), 2017/01/11 20:07:27]

pkasting@:  I was thinking of alternative ways to do this and one thing that I
thought of was to get rid of this constant, have FrameBackground always render
the solid color, change NativeThemeBase::PaintFrameTopArea be a no-op, and have
the NativeThemeGtk3::PaintFrameTopArea override do the actual drawing.  The
disadvantage to this would be redundantly drawing the top area in
FrameBackground on Linux.  wdyt?

[Peter Kasting, 2017/01/12 18:53:53]

I think if I were going to change this, it would be to try to avoid routing
through NativeTheme at all.  It feels weird somehow that we're calling this a
"theme part".  Maybe that's just because of my Windows-centric view of the
world, where this isn't a theme part.  I was imagining
FrameBackground::PaintFrameTopArea() doing this painting directly (via ifdefs),
or directly calling code in platform-specific files that would do this, rather
than indirecting through NativeTheme.  But maybe we can't easily do the right
thing on Linux through that route.

If we need to go through NativeTheme to begin with, then I think the existing
way you've done it is OK.  I wouldn't make the change you propose.

Separately, I find the comment "The theme can ignore this color" confusing. 
NativeTheme doesn't ignore this color, and it's not really clear which "theme"
you're saying can ignore this and why.

[Tom (Use chromium acct), 2017/01/12 21:51:09]

Changed the comment, hopefully this one's better

+  };
+
   struct InnerSpinButtonExtraParams {
     bool spin_up;
     bool read_only;
     int classic_state;  // Used on Windows when uxtheme is not available.
   };
 
   struct MenuArrowExtraParams {
     bool pointing_right;
     // Used for the disabled state to indicate if the item is both disabled and
     // selected.
@@ skipping 78 matching lines @@
   struct TrackbarExtraParams {
     bool vertical;
     int classic_state;  // Used on Windows when uxtheme is not available.
   };
 
   union NATIVE_THEME_EXPORT ExtraParams {
     ExtraParams();
     ExtraParams(const ExtraParams& other);
 
     ButtonExtraParams button;
+    FrameTopAreaExtraParams frame_top_area;
     InnerSpinButtonExtraParams inner_spin;
     MenuArrowExtraParams menu_arrow;
     MenuCheckExtraParams menu_check;
     MenuItemExtraParams menu_item;
     MenuListExtraParams menu_list;
     MenuBackgroundExtraParams menu_background;
     ProgressBarExtraParams progress_bar;
     ScrollbarArrowExtraParams scrollbar_arrow;
     ScrollbarTrackExtraParams scrollbar_track;
     ScrollbarThumbExtraParams scrollbar_thumb;
@@ skipping 167 matching lines @@
  private:
   // Observers to notify when the native theme changes.
   base::ObserverList<NativeThemeObserver> native_theme_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeTheme);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_H_