Fix STR#2600 for the MSWindow and Mac OS platforms.

A new event FL_SCREEN_CONFIGURATION_CHANGED is introduced.
Fl::add_handler() allows to register a callback for this event.
The unix/X11 implementation is still missing.

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@9087 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

FL/Enumerations.H

@@ -283,7 +283,11 @@ enum Fl_Event {	// events
       If the widget returns 1, it will receive the data in the immediately 
       following FL_PASTE event.
    */
-  FL_DND_RELEASE	= 23
+  FL_DND_RELEASE	= 23,
+  /** The screen configuration (number, positions) was changed.
+   Use Fl::add_handler() to be notified of this event.
+   */
+  FL_SCREEN_CONFIGURATION_CHANGED = 24
 };
 
 /** \name When Conditions */

src/Fl.cxx

@@ -776,8 +776,9 @@ static handler_link *handlers = 0;
   them returns non-zero then the event is ignored.  Events that cause
   this to be called are:
 
-  - FL_SHORTCUT events that are not recognized by any widget.
+  - \ref FL_SHORTCUT events that are not recognized by any widget.
     This lets you provide global shortcut keys.
+  - \ref FL_SCREEN_CONFIGURATION_CHANGED events (not implemented on the X11 platform).
   - System events that FLTK does not recognize.  See fl_xevent.
   - \e Some other events when the widget FLTK selected returns
     zero from its handle() method.  Exactly which ones may change

src/Fl_cocoa.mm

@@ -1099,6 +1099,7 @@ void fl_open_callback(void (*cb)(const char *)) {
       [[NSNotificationCenter defaultCenter] postNotificationName:NSWindowDidMoveNotification object:win];
       }
     }
+  Fl::handle(FL_SCREEN_CONFIGURATION_CHANGED, NULL);
 }
 - (void)applicationWillResignActive:(NSNotification *)notify
 {

src/Fl_win32.cxx

@@ -1203,6 +1203,9 @@ static LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lPar
     // unclear on what is correct:
     if (fl_msg.message == WM_RENDERALLFORMATS) CloseClipboard();
     return 1;}
+  case WM_DISPLAYCHANGE: // occurs when screen configuration (number, position) changes
+    Fl::handle(FL_SCREEN_CONFIGURATION_CHANGED, NULL);
+    return 0;
 
   default:
     if (Fl::handle(0,0)) return 0;