* Source/NSWindow.m (sendEvent:): set current screen to ivar and

send WindowDidChangeScreen notification if window was moved to other
  screen.

* Source/NSScreen.m (mainScreen): returns screen of main menu if no
  key window exists. With this implementation default screen (screen at
  index 0 in screens array) should never be returned.

* Source/NSMenu.m (initWithTitle:): observe WindowDidBecomeKey and
  WindowDidChangeScreen notifications.
  (windowDidChangeScreen:): new method to handle observed notification
  added above. Moves menu to screen of key window for both cases: when
  key window moved to or activated on different screen.

ChangeLog

@@ -1,3 +1,19 @@
+2020-02-12  Sergii Stoian  <stoyan255@gmail.com>
+
+	* Source/NSWindow.m (sendEvent:): set current screen to ivar and
+	send WindowDidChangeScreen notification if window was moved to other
+	screen.
+
+	* Source/NSScreen.m (mainScreen): returns screen of main menu if no
+	key window exists. With this implementation default screen (screen at
+	index 0 in screens array) should never be returned.
+
+	* Source/NSMenu.m (initWithTitle:): observe WindowDidBecomeKey and
+	WindowDidChangeScreen notifications.
+	(windowDidChangeScreen:): new method to handle observed notification
+	added above. Moves menu to screen of key window for both cases: when
+	key window moved to or activated on different screen.
+
 2020-02-09 Fred Kiefer <FredKiefer@gmx.de>
 
 	* Source/NSFontDescriptor.m: Correct key for encoding and decoding.

Source/NSMenu.m

@@ -712,6 +712,15 @@ static BOOL menuBarVisible = YES;
       name: NSEnqueuedMenuMoveName
       object: self];
   
+  [nc addObserver: self
+         selector: @selector(windowDidChangeScreen:)
+             name: NSWindowDidBecomeKeyNotification
+           object: nil];
+  [nc addObserver: self
+         selector: @selector(windowDidChangeScreen:)
+             name: NSWindowDidChangeScreenNotification
+           object: nil];
+  
   return self;
 }
 
@@ -1860,6 +1869,36 @@ static BOOL menuBarVisible = YES;
   }
 }
 
+- (void) windowDidChangeScreen: (NSNotification*)notification
+{
+  NSWindow *window = [notification object];
+  NSRect   frame;
+  NSRect   oldScreenFrame;
+  NSRect   newScreenFrame;
+  NSPoint  offset;
+
+  if (window == _aWindow
+      || [window isKeyWindow] == NO
+      || [_aWindow screen] == [window screen]
+      || [_aWindow isVisible] == NO)
+    return;
+
+  oldScreenFrame = [[_aWindow screen] frame];
+  newScreenFrame = [[window screen] frame];
+  frame = [_aWindow frame];
+  
+  // Keep left offset fixed
+  offset.x = frame.origin.x - oldScreenFrame.origin.x;
+  frame.origin.x = newScreenFrame.origin.x + offset.x;
+
+  // Keep top offset fixed
+  offset.y = NSMaxY(oldScreenFrame) - NSMaxY(frame);
+  frame.origin.y = NSMaxY(newScreenFrame) - offset.y - frame.size.height;
+  
+  // setFrame: changes _screen value.
+  [_aWindow setFrame: frame display: NO];
+}
+
 - (BOOL) isTransient
 {
   return _menu.transient;

Source/NSScreen.m

@@ -122,6 +122,11 @@ static NSMutableArray *screenArray = nil;
   NSWindow *keyWindow;
 
   keyWindow = [NSApp keyWindow];
+  if (keyWindow == nil)
+    {
+      keyWindow = [[NSApp mainMenu] window];
+    }
+  
   if (keyWindow != nil)
     {
       return [keyWindow screen];

Source/NSWindow.m

@@ -4180,6 +4180,17 @@ checkCursorRectanglesExited(NSView *theView,  NSEvent *theEvent, NSPoint lastPoi
                 }
               [nc postNotificationName: NSWindowDidMoveNotification
                   object: self];
+              // Check if window moved to an other screen
+              {
+                NSScreen *newScreen = [self _screenForFrame: _frame];
+
+                if (newScreen != _screen)
+                  {
+                    ASSIGN(_screen, newScreen);
+                    [nc postNotificationName: NSWindowDidChangeScreenNotification
+                                      object: self];
+                  }
+              }
               break;
               
             case GSAppKitWindowResized: