macOS platform: compute the screen number where a top window is mapped.

src/Fl_cocoa.mm

@@ -1234,6 +1234,10 @@ static FLWindowDelegate *flwindowdelegate_instance = nil;
     if (window->parent()) [nsw recursivelySendToSubwindows:@selector(checkSubwindowFrame) applyToSelf:YES];
     starting_moved_window = NULL;
   }
+  if (!window->parent()) {
+    int nscreen = Fl::screen_num(window->x(), window->y(), window->w(), window->h());
+    Fl_Window_Driver::driver(window)->screen_num(nscreen);
+  }
   fl_unlock_function();
 }
 - (void)view_did_resize:(NSNotification *)notif

src/drivers/Cocoa/Fl_Cocoa_Window_Driver.H

@@ -85,6 +85,7 @@ private:
   // stores 3 binary flags: whether window is mapped to retina display; whether resolution just changed;
   // whether window's view received the [FLView view_did_resize] message
   unsigned window_flags_;
+  int screen_num_; // number of screen where window is mapped
 public:
   Fl_Cocoa_Window_Driver(Fl_Window*);
   ~Fl_Cocoa_Window_Driver();
@@ -133,6 +134,8 @@ public:
   virtual void size_range();
   virtual void iconize();
   virtual void decoration_sizes(int *top, int *left,  int *right, int *bottom);
+  virtual int screen_num();
+  virtual void screen_num(int n);
   // --- window cursor stuff
   virtual int set_cursor(Fl_Cursor);
   virtual int set_cursor(const Fl_RGB_Image*, int, int);

src/drivers/Cocoa/Fl_Cocoa_Window_Driver.cxx

@@ -51,6 +51,7 @@ Fl_Cocoa_Window_Driver::Fl_Cocoa_Window_Driver(Fl_Window *win)
   cursor = nil;
   window_flags_ = 0;
   icon_image = NULL;
+  screen_num_ = 0;
 }
 
 
@@ -342,3 +343,12 @@ void Fl_Cocoa_Window_Driver::capture_titlebar_and_borders(Fl_RGB_Image*& top, Fl
   top->scale(w(),htop, s <1 ? 0 : 1, 1);
   CGContextRelease(auxgc);
 }
+
+void Fl_Cocoa_Window_Driver::screen_num(int n) {
+  screen_num_ = n;
+}
+
+int Fl_Cocoa_Window_Driver::screen_num() {
+  if (pWindow->parent()) return pWindow->top_window()->screen_num();
+  else return screen_num_;
+}