macOS: Colored Titlebar and WM Window Decoration Styles API - GSoC 2024

As part of the GSoC 2024 project "Improvements to the Blender macOS User
Interface Experience" [1], this patch implements colored titlebar window
decorations on macOS, based on the current Blender theme colors.

Additionally, this patch introduces a new WM/GHOST API for implementing
and enabling custom decoration styles on any system/desktop environment
that support customizing window decorations.

[1]: https://devtalk.blender.org/t/gsoc-2024-proposal-improvements-to-the-blender-macos-user-interface-experience/34022)

Pull Request: https://projects.blender.org/blender/blender/pulls/123982

intern/ghost/GHOST_C-api.h

@@ -593,6 +593,31 @@ extern char *GHOST_GetTitle(GHOST_WindowHandle windowhandle);
  */
 extern GHOST_TSuccess GHOST_SetPath(GHOST_WindowHandle windowhandle, const char *filepath);
 
+/**
+ * Return the current window decoration style flags.
+ */
+extern GHOST_TWindowDecorationStyleFlags GHOST_GetWindowDecorationStyleFlags(
+    GHOST_WindowHandle windowhandle);
+
+/**
+ * Set the window decoration style flags.
+ * \param styleFlags: Window decoration style flags.
+ */
+extern void GHOST_SetWindowDecorationStyleFlags(GHOST_WindowHandle windowhandle,
+                                                GHOST_TWindowDecorationStyleFlags styleFlags);
+
+/**
+ * Set the window decoration style settings.
+ * \param decorationSettings: Window decoration style settings.
+ */
+extern void GHOST_SetWindowDecorationStyleSettings(
+    GHOST_WindowHandle windowhandle, GHOST_WindowDecorationStyleSettings decorationSettings);
+
+/**
+ * Apply the window decoration style using the current flags and settings.
+ */
+extern GHOST_TSuccess GHOST_ApplyWindowDecorationStyle(GHOST_WindowHandle windowhandle);
+
 /**
  * Returns the window rectangle dimensions.
  * These are screen coordinates.

intern/ghost/GHOST_IWindow.hh

@@ -87,6 +87,29 @@ class GHOST_IWindow {
    */
   virtual GHOST_TSuccess setPath(const char *filepath) = 0;
 
+  /**
+   * Return the current window decoration style flags.
+   */
+  virtual GHOST_TWindowDecorationStyleFlags getWindowDecorationStyleFlags() = 0;
+
+  /**
+   * Set the window decoration style flags.
+   * \param styleFlags: Window decoration style flags.
+   */
+  virtual void setWindowDecorationStyleFlags(GHOST_TWindowDecorationStyleFlags styleFlags) = 0;
+
+  /**
+   * Set the window decoration style settings.
+   * \param decorationSettings: Window decoration style settings.
+   */
+  virtual void setWindowDecorationStyleSettings(
+      GHOST_WindowDecorationStyleSettings decorationSettings) = 0;
+
+  /**
+   * Apply the window decoration style using the current flags and settings.
+   */
+  virtual GHOST_TSuccess applyWindowDecorationStyle() = 0;
+
   /**
    * Returns the window rectangle dimensions.
    * These are screen coordinates.

intern/ghost/GHOST_Types.h

@@ -119,6 +119,10 @@ typedef enum {
    * Support detecting the physical trackpad direction.
    */
   GHOST_kCapabilityTrackpadPhysicalDirection = (1 << 7),
+  /**
+   * Support for window decoration styles.
+   */
+  GHOST_kCapabilityWindowDecorationStyles = (1 << 8),
 } GHOST_TCapabilityFlag;
 
 /**
@@ -129,7 +133,8 @@ typedef enum {
   (GHOST_kCapabilityCursorWarp | GHOST_kCapabilityWindowPosition | \
    GHOST_kCapabilityPrimaryClipboard | GHOST_kCapabilityGPUReadFrontBuffer | \
    GHOST_kCapabilityClipboardImages | GHOST_kCapabilityDesktopSample | \
-   GHOST_kCapabilityInputIME | GHOST_kCapabilityTrackpadPhysicalDirection)
+   GHOST_kCapabilityInputIME | GHOST_kCapabilityTrackpadPhysicalDirection | \
+   GHOST_kCapabilityWindowDecorationStyles)
 
 /* Xtilt and Ytilt represent how much the pen is tilted away from
  * vertically upright in either the X or Y direction, with X and Y the
@@ -694,6 +699,11 @@ typedef enum {
   /* Can be extended as needed. */
 } GHOST_TUserSpecialDirTypes;
 
+typedef enum {
+  GHOST_kDecorationNone = 0,
+  GHOST_kDecorationColoredTitleBar = (1 << 0),
+} GHOST_TWindowDecorationStyleFlags;
+
 typedef struct {
   /** Number of pixels on a line. */
   uint32_t xPixels;
@@ -720,6 +730,11 @@ typedef struct {
   GHOST_GPUDevice preferred_device;
 } GHOST_GPUSettings;
 
+typedef struct {
+  float colored_titlebar_bg_color[3];
+  float colored_titlebar_fg_color[3];
+} GHOST_WindowDecorationStyleSettings;
+
 #ifdef WITH_VULKAN_BACKEND
 typedef struct {
   /** Image handle to the image that will be presented to the user. */

intern/ghost/intern/GHOST_C-api.cc

@@ -606,6 +606,33 @@ GHOST_TSuccess GHOST_SetPath(GHOST_WindowHandle windowhandle, const char *filepa
   return window->setPath(filepath);
 }
 
+GHOST_TWindowDecorationStyleFlags GHOST_GetWindowDecorationStyleFlags(
+    GHOST_WindowHandle windowhandle)
+{
+  GHOST_IWindow *window = (GHOST_IWindow *)windowhandle;
+  return window->getWindowDecorationStyleFlags();
+}
+
+void GHOST_SetWindowDecorationStyleFlags(GHOST_WindowHandle windowhandle,
+                                         GHOST_TWindowDecorationStyleFlags styleFlags)
+{
+  GHOST_IWindow *window = (GHOST_IWindow *)windowhandle;
+  window->setWindowDecorationStyleFlags(styleFlags);
+}
+
+void GHOST_SetWindowDecorationStyleSettings(GHOST_WindowHandle windowhandle,
+                                            GHOST_WindowDecorationStyleSettings decorationSettings)
+{
+  GHOST_IWindow *window = (GHOST_IWindow *)windowhandle;
+  window->setWindowDecorationStyleSettings(decorationSettings);
+}
+
+GHOST_TSuccess GHOST_ApplyWindowDecorationStyle(GHOST_WindowHandle windowhandle)
+{
+  GHOST_IWindow *window = (GHOST_IWindow *)windowhandle;
+  return window->applyWindowDecorationStyle();
+}
+
 GHOST_RectangleHandle GHOST_GetWindowBounds(GHOST_WindowHandle windowhandle)
 {
   const GHOST_IWindow *window = (const GHOST_IWindow *)windowhandle;

intern/ghost/intern/GHOST_SystemHeadless.hh

@@ -54,7 +54,8 @@ class GHOST_SystemHeadless : public GHOST_System {
                                  ~(GHOST_kCapabilityWindowPosition | GHOST_kCapabilityCursorWarp |
                                    GHOST_kCapabilityPrimaryClipboard |
                                    GHOST_kCapabilityDesktopSample |
-                                   GHOST_kCapabilityClipboardImages | GHOST_kCapabilityInputIME));
+                                   GHOST_kCapabilityClipboardImages | GHOST_kCapabilityInputIME |
+                                   GHOST_kCapabilityWindowDecorationStyles));
   }
   char *getClipboard(bool /*selection*/) const override
   {

intern/ghost/intern/GHOST_SystemSDL.cc

@@ -791,7 +791,9 @@ GHOST_TCapabilityFlag GHOST_SystemSDL::getCapabilities() const
           /* This SDL back-end has not yet implemented image copy/paste. */
           GHOST_kCapabilityClipboardImages |
           /* No support yet for IME input methods. */
-          GHOST_kCapabilityInputIME));
+          GHOST_kCapabilityInputIME |
+          /* No support for window decoration styles. */
+          GHOST_kCapabilityWindowDecorationStyles));
 }
 
 char *GHOST_SystemSDL::getClipboard(bool /*selection*/) const

intern/ghost/intern/GHOST_SystemWayland.cc

@@ -8730,6 +8730,10 @@ GHOST_TCapabilityFlag GHOST_SystemWayland::getCapabilities() const
           GHOST_kCapabilityGPUReadFrontBuffer |
           /* This WAYLAND back-end has not yet implemented desktop color sample. */
           GHOST_kCapabilityDesktopSample |
+          /* This WAYLAND back-end doesn't have support for window decoration styles.
+           * In all likelihood, this back-end will eventually need to support client-side
+           * decorations, see #113795. */
+          GHOST_kCapabilityWindowDecorationStyles |
           /* This flag will eventually be removed. */
           ((has_wl_trackpad_physical_direction == 1) ?
                0 :

intern/ghost/intern/GHOST_SystemWin32.cc

@@ -586,7 +586,10 @@ GHOST_TCapabilityFlag GHOST_SystemWin32::getCapabilities() const
   return GHOST_TCapabilityFlag(GHOST_CAPABILITY_FLAG_ALL &
                                ~(
                                    /* WIN32 has no support for a primary selection clipboard. */
-                                   GHOST_kCapabilityPrimaryClipboard));
+                                   GHOST_kCapabilityPrimaryClipboard |
+                                   /* This WIN32 backend has not yet implemented support for window
+                                    * decoration styles. */
+                                   GHOST_kCapabilityWindowDecorationStyles));
 }
 
 GHOST_TSuccess GHOST_SystemWin32::init()

intern/ghost/intern/GHOST_SystemX11.cc

@@ -1817,7 +1817,9 @@ GHOST_TCapabilityFlag GHOST_SystemX11::getCapabilities() const
                                    /* No support yet for image copy/paste. */
                                    GHOST_kCapabilityClipboardImages |
                                    /* No support yet for IME input methods. */
-                                   GHOST_kCapabilityInputIME));
+                                   GHOST_kCapabilityInputIME |
+                                   /* No support for window decoration styles. */
+                                   GHOST_kCapabilityWindowDecorationStyles));
 }
 
 void GHOST_SystemX11::addDirtyWindow(GHOST_WindowX11 *bad_wind)

intern/ghost/intern/GHOST_Window.cc

@@ -32,6 +32,8 @@ GHOST_Window::GHOST_Window(uint32_t width,
       m_progressBarVisible(false),
       m_canAcceptDragOperation(false),
       m_isUnsavedChanges(false),
+      m_windowDecorationStyleFlags(GHOST_kDecorationNone),
+      m_windowDecorationStyleSettings(),
       m_wantStereoVisual(wantStereoVisual),
       m_nativePixelSize(1.0f),
       m_context(new GHOST_ContextNone(false))
@@ -55,6 +57,22 @@ void *GHOST_Window::getOSWindow() const
   return nullptr;
 }
 
+GHOST_TWindowDecorationStyleFlags GHOST_Window::getWindowDecorationStyleFlags()
+{
+  return m_windowDecorationStyleFlags;
+}
+
+void GHOST_Window::setWindowDecorationStyleFlags(GHOST_TWindowDecorationStyleFlags styleFlags)
+{
+  m_windowDecorationStyleFlags = styleFlags;
+}
+
+void GHOST_Window::setWindowDecorationStyleSettings(
+    GHOST_WindowDecorationStyleSettings decorationSettings)
+{
+  m_windowDecorationStyleSettings = decorationSettings;
+}
+
 GHOST_TSuccess GHOST_Window::setDrawingContextType(GHOST_TDrawingContextType type)
 {
   if (type != m_drawingContextType) {

intern/ghost/intern/GHOST_Window.hh

@@ -89,6 +89,33 @@ class GHOST_Window : public GHOST_IWindow {
     return GHOST_kFailure;
   }
 
+  /**
+   * Return the current window decoration style flags.
+   */
+  virtual GHOST_TWindowDecorationStyleFlags getWindowDecorationStyleFlags() override;
+
+  /**
+   * Set the window decoration style flags.
+   * \param styleFlags: Window decoration style flags.
+   */
+  virtual void setWindowDecorationStyleFlags(
+      GHOST_TWindowDecorationStyleFlags styleFlags) override;
+
+  /**
+   * Set the window decoration style settings.
+   * \param decorationSettings: Window decoration style settings.
+   */
+  virtual void setWindowDecorationStyleSettings(
+      GHOST_WindowDecorationStyleSettings decorationSettings) override;
+
+  /**
+   * Apply the window decoration style using the current flags and settings.
+   */
+  virtual GHOST_TSuccess applyWindowDecorationStyle() override
+  {
+    return GHOST_kSuccess;
+  }
+
   /**
    * Returns the current cursor shape.
    * \return The current cursor shape.
@@ -410,6 +437,10 @@ class GHOST_Window : public GHOST_IWindow {
   /** Stores whether this is a full screen window. */
   bool m_fullScreen;
 
+  /** Window Decoration Styles. */
+  GHOST_TWindowDecorationStyleFlags m_windowDecorationStyleFlags;
+  GHOST_WindowDecorationStyleSettings m_windowDecorationStyleSettings;
+
   /** Whether to attempt to initialize a context with a stereo frame-buffer. */
   bool m_wantStereoVisual;
 

intern/ghost/intern/GHOST_WindowCocoa.hh

@@ -93,6 +93,11 @@ class GHOST_WindowCocoa : public GHOST_Window {
    */
   GHOST_TSuccess setPath(const char *filepath) override;
 
+  /**
+   * Apply the window decoration style using the current flags and settings.
+   */
+  GHOST_TSuccess applyWindowDecorationStyle() override;
+
   /**
    * Returns the window rectangle dimensions.
    * The dimensions are given in screen coordinates that are

intern/ghost/intern/GHOST_WindowCocoa.mm

@@ -565,6 +565,38 @@ GHOST_TSuccess GHOST_WindowCocoa::setPath(const char *filepath)
   return GHOST_kSuccess;
 }
 
+GHOST_TSuccess GHOST_WindowCocoa::applyWindowDecorationStyle()
+{
+  @autoreleasepool {
+    if (m_windowDecorationStyleFlags & GHOST_kDecorationColoredTitleBar) {
+      const float *background_color = m_windowDecorationStyleSettings.colored_titlebar_bg_color;
+
+      /* Titlebar background color. */
+      m_window.backgroundColor = [NSColor colorWithRed:background_color[0]
+                                                 green:background_color[1]
+                                                  blue:background_color[2]
+                                                 alpha:1.0];
+
+      /* Titlebar foreground color.
+       * Use the value component of the titlebar background's HSV representation to determine
+       * whether we should use the macOS dark or light titlebar text appearance. With values below
+       * 0.5 considered as dark themes, and values above 0.5 considered as light themes.
+       */
+      const float hsv_v = MAX(background_color[0], MAX(background_color[1], background_color[2]));
+
+      const NSAppearanceName win_appearance = hsv_v > 0.5 ? NSAppearanceNameVibrantLight :
+                                                            NSAppearanceNameVibrantDark;
+
+      m_window.appearance = [NSAppearance appearanceNamed:win_appearance];
+      m_window.titlebarAppearsTransparent = YES;
+    }
+    else {
+      m_window.titlebarAppearsTransparent = NO;
+    }
+  }
+  return GHOST_kSuccess;
+}
+
 void GHOST_WindowCocoa::getWindowBounds(GHOST_Rect &bounds) const
 {
   GHOST_ASSERT(getValid(), "GHOST_WindowCocoa::getWindowBounds(): window invalid");

source/blender/editors/screen/area.cc

@@ -2720,6 +2720,13 @@ void ED_area_newspace(bContext *C, ScrArea *area, int type, const bool skip_regi
     WM_window_title(CTX_wm_manager(C), CTX_wm_window(C));
   }
 
+  /* If window decoration styles are supported, send a notification to re-apply them. */
+  /* TODO: The `bl_animation_keyframing` test fails here if WM_capabilities_flags is called in
+   *       background mode. Remove the !G.background check once the test has been fixed. */
+  if (!G.background && WM_capabilities_flag() & WM_CAPABILITY_WINDOW_DECORATION_STYLES) {
+    WM_event_add_notifier(C, NC_WINDOW, nullptr);
+  }
+
   /* also redraw when re-used */
   ED_area_tag_redraw(area);
 }

source/blender/editors/screen/screen_edit.cc

@@ -660,10 +660,16 @@ void ED_screen_do_listen(bContext *C, const wmNotifier *note)
       }
       break;
     case NC_WINDOW:
+      if (WM_capabilities_flag() & WM_CAPABILITY_WINDOW_DECORATION_STYLES) {
+        WM_window_apply_decoration_style(win, screen);
+      }
       screen->do_draw = true;
       break;
     case NC_SCREEN:
       if (note->action == NA_EDITED) {
+        if (WM_capabilities_flag() & WM_CAPABILITY_WINDOW_DECORATION_STYLES) {
+          WM_window_apply_decoration_style(win, screen);
+        }
         screen->do_draw = screen->do_refresh = true;
       }
       break;
@@ -1312,7 +1318,7 @@ void ED_screen_global_areas_refresh(wmWindow *win)
 {
   /* Don't create global area for child and temporary windows. */
   bScreen *screen = BKE_workspace_active_screen_get(win->workspace_hook);
-  if ((win->parent != nullptr) || screen->temp) {
+  if (!WM_window_is_main_top_level(win)) {
     if (win->global_areas.areabase.first) {
       screen->do_refresh = true;
       BKE_screen_area_map_free(&win->global_areas);

source/blender/windowmanager/WM_api.hh

@@ -185,10 +185,12 @@ enum eWM_CapabilitiesFlag {
   WM_CAPABILITY_INPUT_IME = (1 << 6),
   /** Trackpad physical scroll detection. */
   WM_CAPABILITY_TRACKPAD_PHYSICAL_DIRECTION = (1 << 7),
+  /** Support for window decoration styles. */
+  WM_CAPABILITY_WINDOW_DECORATION_STYLES = (1 << 8),
   /** The initial value, indicates the value needs to be set by inspecting GHOST. */
   WM_CAPABILITY_INITIALIZED = (1u << 31),
 };
-ENUM_OPERATORS(eWM_CapabilitiesFlag, WM_CAPABILITY_TRACKPAD_PHYSICAL_DIRECTION)
+ENUM_OPERATORS(eWM_CapabilitiesFlag, WM_CAPABILITY_WINDOW_DECORATION_STYLES)
 
 eWM_CapabilitiesFlag WM_capabilities_flag();
 
@@ -284,6 +286,7 @@ void WM_window_rect_calc(const wmWindow *win, rcti *r_rect);
  * \note Depends on #UI_SCALE_FAC. Should that be outdated, call #WM_window_set_dpi first.
  */
 void WM_window_screen_rect_calc(const wmWindow *win, rcti *r_rect);
+bool WM_window_is_main_top_level(const wmWindow *win);
 bool WM_window_is_fullscreen(const wmWindow *win);
 bool WM_window_is_maximized(const wmWindow *win);
 
@@ -374,6 +377,29 @@ void WM_window_title(wmWindowManager *wm, wmWindow *win, const char *title = nul
 
 bool WM_stereo3d_enabled(wmWindow *win, bool skip_stereo3d_check);
 
+/* Window Decoration Styles. */
+
+/* Flags for #WM_window_decoration_set_style().
+ * NOTE: To be kept in sync with #GHOST_TWindowDecorationFlags. */
+enum eWM_WindowDecorationStyleFlag {
+  /** No decoration styling. */
+  WM_WINDOW_DECORATION_STYLE_NONE = 0,
+  /** Colored Titlebar. */
+  WM_WINDOW_DECORATION_STYLE_COLORED_TITLEBAR = (1 << 0),
+};
+ENUM_OPERATORS(eWM_WindowDecorationStyleFlag, WM_WINDOW_DECORATION_STYLE_COLORED_TITLEBAR)
+
+/* Get/set window decoration style flags. */
+eWM_WindowDecorationStyleFlag WM_window_get_decoration_style_flags(const wmWindow *win);
+void WM_window_set_decoration_style_flags(const wmWindow *win,
+                                          eWM_WindowDecorationStyleFlag style_flags);
+/* Apply the window decoration style using the current style flags and by parsing style
+ * settings from the current Blender theme.
+ * The screen parameter is optional, and can be passed for enhanced theme parsing.
+ * NOTE: Avoid calling this function directly, prefer sending an NC_WINDOW WM notification instead.
+ */
+void WM_window_apply_decoration_style(const wmWindow *win, const bScreen *screen = nullptr);
+
 /* `wm_files.cc`. */
 
 void WM_file_autoexec_init(const char *filepath);

source/blender/windowmanager/intern/wm_window.cc

@@ -598,6 +598,72 @@ void WM_window_set_dpi(const wmWindow *win)
   U.widget_unit = int(roundf(18.0f * U.scale_factor)) + (2 * pixelsize);
 }
 
+eWM_WindowDecorationStyleFlag WM_window_get_decoration_style_flags(const wmWindow *win)
+{
+  const GHOST_TWindowDecorationStyleFlags ghost_style_flags = GHOST_GetWindowDecorationStyleFlags(
+      static_cast<GHOST_WindowHandle>(win->ghostwin));
+
+  eWM_WindowDecorationStyleFlag wm_style_flags = WM_WINDOW_DECORATION_STYLE_NONE;
+
+  if (ghost_style_flags & GHOST_kDecorationColoredTitleBar) {
+    wm_style_flags |= WM_WINDOW_DECORATION_STYLE_COLORED_TITLEBAR;
+  }
+
+  return wm_style_flags;
+}
+
+void WM_window_set_decoration_style_flags(const wmWindow *win,
+                                          eWM_WindowDecorationStyleFlag style_flags)
+{
+  unsigned int ghost_style_flags = GHOST_kDecorationNone;
+
+  if (style_flags & WM_WINDOW_DECORATION_STYLE_COLORED_TITLEBAR) {
+    ghost_style_flags |= GHOST_kDecorationColoredTitleBar;
+  }
+
+  GHOST_SetWindowDecorationStyleFlags(
+      static_cast<GHOST_WindowHandle>(win->ghostwin),
+      static_cast<GHOST_TWindowDecorationStyleFlags>(ghost_style_flags));
+}
+
+static void wm_window_decoration_style_set_from_theme(const wmWindow *win, const bScreen *screen)
+{
+  /* Set the decoration style settings from the current theme colors.
+   * NOTE: screen may be null. In which case, only the window is used as a theme provider. */
+  GHOST_WindowDecorationStyleSettings decoration_settings = {};
+
+  /* Colored Titlebar Decoration. */
+  /* For main windows, use the topbar color. */
+  if (WM_window_is_main_top_level(win)) {
+    UI_SetTheme(SPACE_TOPBAR, RGN_TYPE_HEADER);
+  }
+  /* For single editor floating windows, use the editor header color. */
+  else if (screen && BLI_listbase_is_single(&screen->areabase)) {
+    const ScrArea *main_area = static_cast<ScrArea *>(screen->areabase.first);
+    UI_SetTheme(main_area->spacetype, RGN_TYPE_HEADER);
+  }
+  /* For floating window with multiple editors/areas, use the default space color. */
+  else {
+    UI_SetTheme(0, RGN_TYPE_WINDOW);
+  }
+
+  float titlebar_bg_color[3], titlebar_fg_color[3];
+  UI_GetThemeColor3fv(TH_BACK, titlebar_bg_color);
+  UI_GetThemeColor3fv(TH_BUTBACK_TEXT, titlebar_fg_color);
+  copy_v3_v3(decoration_settings.colored_titlebar_bg_color, titlebar_bg_color);
+  copy_v3_v3(decoration_settings.colored_titlebar_fg_color, titlebar_fg_color);
+
+  GHOST_SetWindowDecorationStyleSettings(static_cast<GHOST_WindowHandle>(win->ghostwin),
+                                         decoration_settings);
+}
+
+void WM_window_apply_decoration_style(const wmWindow *win, const bScreen *screen)
+{
+  BLI_assert(WM_capabilities_flag() & WM_CAPABILITY_WINDOW_DECORATION_STYLES);
+  wm_window_decoration_style_set_from_theme(win, screen);
+  GHOST_ApplyWindowDecorationStyle(static_cast<GHOST_WindowHandle>(win->ghostwin));
+}
+
 /**
  * When windows are activated, simulate modifier press/release to match the current state of
  * held modifier keys, see #40317.
@@ -857,6 +923,12 @@ static void wm_window_ghostwindow_ensure(wmWindowManager *wm, wmWindow *win, boo
     wm_window_ensure_eventstate(win);
 
     WM_window_set_dpi(win);
+
+    if (WM_capabilities_flag() & WM_CAPABILITY_WINDOW_DECORATION_STYLES) {
+      /* Only decoration style we have for now. */
+      WM_window_set_decoration_style_flags(win, WM_WINDOW_DECORATION_STYLE_COLORED_TITLEBAR);
+      WM_window_apply_decoration_style(win);
+    }
   }
 
   /* Add key-map handlers (1 handler for all keys in map!). */
@@ -2174,6 +2246,9 @@ eWM_CapabilitiesFlag WM_capabilities_flag()
   if (ghost_flag & GHOST_kCapabilityTrackpadPhysicalDirection) {
     flag |= WM_CAPABILITY_TRACKPAD_PHYSICAL_DIRECTION;
   }
+  if (ghost_flag & GHOST_kCapabilityWindowDecorationStyles) {
+    flag |= WM_CAPABILITY_WINDOW_DECORATION_STYLES;
+  }
 
   return flag;
 }
@@ -2808,6 +2883,19 @@ bool WM_window_is_maximized(const wmWindow *win)
   return win->windowstate == GHOST_kWindowStateMaximized;
 }
 
+bool WM_window_is_main_top_level(const wmWindow *win)
+{
+  /**
+   * Return whether the window is a main/top-level window. In which case it is expected to contain
+   * global areas (topbar/statusbar).
+   */
+  const bScreen *screen = BKE_workspace_active_screen_get(win->workspace_hook);
+  if ((win->parent != nullptr) || screen->temp) {
+    return false;
+  }
+  return true;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */