Initial framework for dynamic color support

- Fl_Dynamic_Color enum
- Fl::dynamic_color() APIs
- Fl_System_Driver::dynamic_color() API
- FL_DYNAMIC_COLOR_CHANGED event

- Add method for querying dark mode to macOS system and screen drivers.
- Check/update mode whenever we get a layout change from macOS.

- Add detection of dark mode on macOS by using the effectiveAppearance
  property.

- Force redraw of all windows when changing the mode.

- Update Cocoa set_system_colors method to override colors when dynamic
  mode isn't off.

- Add (temporary) support for FL_DYNAMIC_COLOR environment variable to
  override default "off" mode for testing.

To do: add dark mode detection on other platforms than macOS.

Note: Branch 'darkmode' rebased, squashed, and extended
  by Albrecht-S on Dec 17, 2023.

FL/Enumerations.H

@@ -407,7 +407,11 @@ enum Fl_Event { // events
   /** A zoom event (ctrl/+/-/0/ or cmd/+/-/0/) was processed.
    Use Fl::add_handler() to be notified of this event.
    */
-  FL_ZOOM_EVENT = 27
+  FL_ZOOM_EVENT = 27,
+  /** The current dynamic color mode has changed.
+   Call Fl::dynamic_color() to get the new mode.
+   */
+  FL_DYNAMIC_COLOR_CHANGED = 28
 // DEV NOTE: Keep this list in sync with FL/names.h
 };
 
@@ -1380,4 +1384,26 @@ enum Fl_Orientation {
   FL_ORIENT_SE    = 0x07  ///< GUI element pointing SE    (315°)
 };
 
+/**
+    Modern operating systems provide a pair of color themes suitable for use in
+    light and dark ambient lighting. Automatic switching is normally provided
+    either using the current time of day or with ambient lighting sensors.
+    This enumeration lists the values for the Fl::dynamic_color() API, allowing
+    applications to opt in to automatic color theme switching or specify that
+    color switching is not desired.
+
+    \see Fl::dynamic_color()
+ */
+
+enum Fl_Dynamic_Color { // dynamic color values
+  /** Dynamic color themes are off/not supported/disabled */
+  FL_DYNAMIC_COLOR_OFF,
+  /** Automatically change color themes */
+  FL_DYNAMIC_COLOR_AUTO,
+  /** Use the light color theme */
+  FL_DYNAMIC_COLOR_LIGHT,
+  /** Use the dark color theme */
+  FL_DYNAMIC_COLOR_DARK
+};
+
 #endif

FL/Fl.H

@@ -148,6 +148,8 @@ private:
 
 public:
 
+  static Fl_Dynamic_Color dynamic_color_;
+
   static Fl_Screen_Driver *screen_driver();
   static Fl_System_Driver *system_driver();
 #ifdef __APPLE__ // deprecated in 1.4 - only for compatibility with 1.3
@@ -392,6 +394,27 @@ public:
   static void background(uchar, uchar, uchar);
   static void background2(uchar, uchar, uchar);
 
+  // dynamic color:
+  /** Set the dynamic color theme mode.
+
+    Most applications should just call Fl::dynamic_color(FL_DYNAMIC_COLOR_AUTO)
+    to use the user's preferred color theme. The default value is
+    FL_DYNAMIC_COLOR_OFF for compatibility with prior versions of FLTK.
+
+    When set to FL_DYNAMIC_COLOR_AUTO, FLTK will monitor for appearance/theme
+    changes. When the color theme changes, a FL_DYNAMIC_COLOR_CHANGED event is
+    sent to allow widgets to update their colors as needed.
+    */
+  static void dynamic_color(Fl_Dynamic_Color mode);
+  /** Gets the current dynamic color theme.
+
+    Returns FL_DYNAMIC_COLOR_OFF if dynamic color themes are off or disabled
+    (the default), FL_DYNAMIC_COLOR_LIGHT if the current color theme is for use
+    in bright ambient light, or FL_DYNAMIC_COLOR_DARK is the current color
+    theme is for use in dark ambient light.
+    */
+  static Fl_Dynamic_Color dynamic_color();
+
   // schemes:
   static int scheme(const char *name);
   /** See void scheme(const char *name) */

FL/names.h

@@ -73,7 +73,7 @@ const char * const fl_eventnames[] =
   "FL_FULLSCREEN",
   "FL_ZOOM_GESTURE",
   "FL_ZOOM_EVENT",
-  "FL_EVENT_28", // not yet defined, just in case it /will/ be defined ...
+  "FL_DYNAMIC_COLOR_CHANGED",
   "FL_EVENT_29", // not yet defined, just in case it /will/ be defined ...
   "FL_EVENT_30"  // not yet defined, just in case it /will/ be defined ...
 };

src/Fl_System_Driver.H

@@ -222,6 +222,7 @@ public:
   virtual const char *alt_name() { return "Alt"; }
   virtual const char *control_name() { return "Ctrl"; }
   virtual Fl_Sys_Menu_Bar_Driver *sys_menu_bar_driver() { return NULL; }
+  virtual Fl_Dynamic_Color dynamic_color() { return FL_DYNAMIC_COLOR_LIGHT; }
   virtual void lock_ring() {}
   virtual void unlock_ring() {}
   virtual double wait(double);                             // must FL_OVERRIDE

src/Fl_cocoa.mm

@@ -580,6 +580,7 @@ void Fl_Cocoa_Screen_Driver::breakMacEventLoop()
 - (BOOL)process_keydown:(NSEvent*)theEvent;
 - (id)initWithFrame:(NSRect)frameRect;
 - (void)drawRect:(NSRect)rect;
+- (void)layout;
 - (BOOL)acceptsFirstResponder;
 - (BOOL)acceptsFirstMouse:(NSEvent*)theEvent;
 - (void)resetCursorRects;
@@ -2415,6 +2416,38 @@ static FLTextInputContext* fltextinputcontext_instance = nil;
   fl_unlock_function();
 }
 
+- (void)layout
+{
+  // Check to see whether the system appearance has changed...
+  if (Fl::dynamic_color_ == FL_DYNAMIC_COLOR_AUTO) {
+    Fl_Dynamic_Color mode = FL_DYNAMIC_COLOR_LIGHT;
+
+    if (@available(macOS 10.14, *)) {
+      NSAppearanceName temp = [self.effectiveAppearance bestMatchFromAppearancesWithNames:@[
+	  NSAppearanceNameAqua,
+	  NSAppearanceNameDarkAqua
+      ]];
+
+      if ([temp isEqualToString:NSAppearanceNameDarkAqua])
+        mode = FL_DYNAMIC_COLOR_DARK;
+    }
+
+    Fl_Darwin_System_Driver *s = (Fl_Darwin_System_Driver *)Fl::system_driver();
+    if (s->dynamic_color() != mode) {
+      // Change the color mode and force all windows to redraw...
+      s->dynamic_color(mode);
+      Fl::get_system_colors();
+
+      Fl_Window *window;
+      for (window = Fl::first_window(); window; window = Fl::next_window(window))
+        window->redraw();
+    }
+  }
+
+  [super layout];
+}
+
+
 - (BOOL)acceptsFirstResponder
 {
   return [[self window] parentWindow] ? NO : YES; // 10.2

src/Fl_get_system_colors.cxx

@@ -106,6 +106,27 @@ void Fl::get_system_colors()
   Fl::screen_driver()->get_system_colors();
 }
 
+#ifndef FL_DOXYGEN
+Fl_Dynamic_Color Fl::dynamic_color_ = FL_DYNAMIC_COLOR_OFF;
+#endif // !FL_DOXYGEN
+
+void Fl::dynamic_color(Fl_Dynamic_Color mode)
+{
+  dynamic_color_ = mode;
+}
+
+Fl_Dynamic_Color Fl::dynamic_color()
+{
+  // TODO: Remove this
+  if (getenv("FL_DYNAMIC_COLOR") && dynamic_color_ == FL_DYNAMIC_COLOR_OFF)
+    dynamic_color_ = FL_DYNAMIC_COLOR_AUTO;
+
+  if (dynamic_color_ == FL_DYNAMIC_COLOR_AUTO)
+    return Fl::system_driver()->dynamic_color();
+  else
+    return dynamic_color_;
+}
+
 
 //// Simple implementation of 2.0 Fl::scheme() interface...
 #define D1 BORDER_WIDTH

src/drivers/Cocoa/Fl_Cocoa_Screen_Driver.cxx

@@ -201,7 +201,11 @@ static void set_selection_color(uchar r, uchar g, uchar b)
 }
 
 
-// MacOS X currently supports two color schemes - Blue and Graphite.
+// macOS supports two basic color schemes - Light and Dark - with accent and
+// highlight colors.  Older versions limited the accent colors to Blue and
+// Graphite but now you can pick from a rainbow of colors plus the old Graphite
+// gray.
+//
 // Since we aren't emulating the Aqua interface (even if Apple would
 // let us), we use some defaults that are similar to both.  The
 // Fl::scheme("plastic") color/box scheme provides a usable Aqua-like
@@ -212,13 +216,30 @@ void Fl_Cocoa_Screen_Driver::get_system_colors()
 
   Fl_Screen_Driver::get_system_colors();
 
-  if (!bg2_set) Fl::background2(0xff, 0xff, 0xff);
+  Fl_Dynamic_Color mode = Fl::dynamic_color();
-  if (!fg_set) Fl::foreground(0, 0, 0);
+
-  if (!bg_set) Fl::background(0xd8, 0xd8, 0xd8);
+  if (!bg2_set || mode != FL_DYNAMIC_COLOR_OFF) {
+    if (mode == FL_DYNAMIC_COLOR_DARK)
+      Fl::background2(23, 23, 23);
+    else
+      Fl::background2(0xff, 0xff, 0xff);
+  }
+  if (!fg_set || mode != FL_DYNAMIC_COLOR_OFF) {
+    if (mode == FL_DYNAMIC_COLOR_DARK)
+      Fl::foreground(223, 223, 223);
+    else
+      Fl::foreground(0, 0, 0);
+  }
+  if (!bg_set || mode != FL_DYNAMIC_COLOR_OFF) {
+    if (mode == FL_DYNAMIC_COLOR_DARK)
+      Fl::background(50, 50, 50);
+    else
+      Fl::background(0xd8, 0xd8, 0xd8);
+  }
 
 #if 0
   // this would be the correct code, but it does not run on all versions
-  // of OS X. Also, setting a bright selection color would require
+  // of macOS. Also, setting a bright selection color would require
   // some updates in Fl_Adjuster and Fl_Help_Browser
   OSStatus err;
   RGBColor c;
@@ -229,6 +250,7 @@ void Fl_Cocoa_Screen_Driver::get_system_colors()
     set_selection_color(c.red, c.green, c.blue);
 #else
   set_selection_color(0x00, 0x00, 0x80);
+  //set_selection_color(0, 87, 207);
 #endif
 }
 

src/drivers/Darwin/Fl_Darwin_System_Driver.H

@@ -40,6 +40,8 @@
 
 class Fl_Darwin_System_Driver : public Fl_Posix_System_Driver
 {
+  Fl_Dynamic_Color dynamic_color_;
+
 public:
   Fl_Darwin_System_Driver();
   int single_arg(const char *arg) FL_OVERRIDE;
@@ -82,6 +84,8 @@ public:
   Fl_Sys_Menu_Bar_Driver *sys_menu_bar_driver() FL_OVERRIDE;
   double wait(double time_to_wait) FL_OVERRIDE;
   int ready() FL_OVERRIDE;
+  Fl_Dynamic_Color dynamic_color() FL_OVERRIDE;
+  void dynamic_color(Fl_Dynamic_Color mode) { dynamic_color_ = mode; }
 };
 
 #endif // FL_DARWIN_SYSTEM_DRIVER_H

src/drivers/Darwin/Fl_Darwin_System_Driver.cxx

@@ -51,8 +51,9 @@ const char *Fl_Darwin_System_Driver::control_name() {
 
 Fl_Darwin_System_Driver::Fl_Darwin_System_Driver() : Fl_Posix_System_Driver() {
   if (fl_mac_os_version == 0) fl_mac_os_version = calc_mac_os_version();
-  command_key = FL_META;
+  command_key    = FL_META;
-  control_key = FL_CTRL;
+  control_key    = FL_CTRL;
+  dynamic_color_ = FL_DYNAMIC_COLOR_LIGHT;
 }
 
 int Fl_Darwin_System_Driver::single_arg(const char *arg) {
@@ -404,3 +405,7 @@ Fl_Pixmap *Fl_Darwin_System_Driver::tree_closepixmap() {
 int Fl_Darwin_System_Driver::tree_connector_style() {
   return FL_TREE_CONNECTOR_NONE;
 }
+
+Fl_Dynamic_Color Fl_Darwin_System_Driver::dynamic_color() {
+  return dynamic_color_;
+}