Index: macosx/tkMacOSXMouseEvent.c
==================================================================
--- macosx/tkMacOSXMouseEvent.c
+++ macosx/tkMacOSXMouseEvent.c
@@ -46,19 +46,21 @@
  */
 
 @implementation TKApplication(TKMouseEvent)
 - (NSEvent *) tkProcessMouseEvent: (NSEvent *) theEvent
 {
-#ifdef TK_MAC_DEBUG_EVENTS
-    TKLog(@"-[%@(%p) %s] %@", [self class], self, _cmd, theEvent);
-#endif
     NSWindow*    eventWindow = [theEvent window];
     NSEventType	 eventType = [theEvent type];
+    TkWindow *winPtr, *grabWinPtr;
+    Tk_Window tkwin;
 #if 0
     NSTrackingArea  *trackingArea = nil;
     NSInteger eventNumber, clickCount, buttonNumber;
 #endif
+#ifdef TK_MAC_DEBUG_EVENTS
+    TKLog(@"-[%@(%p) %s] %@", [self class], self, _cmd, theEvent);
+#endif
     switch (eventType) {
     case NSMouseEntered:
     case NSMouseExited:
     case NSCursorUpdate:
     case NSLeftMouseDown:
@@ -86,55 +88,98 @@
 	}
 	_windowWithMouse = eventWindow;
 	[_windowWithMouse retain];
     }
 
-    /* Create an Xevent to add to the Tk queue. */
+    /*
+     * Compute the mouse position in Tk screen coordinates (global) and in
+     * the Tk coordinates of its containing Tk Window.
+     */
+
     NSPoint global, local = [theEvent locationInWindow];
-    if (eventWindow) { /* local will be in window coordinates. */
+
+    /*
+     * If the event has no NSWindow, try using the cached NSWindow from the
+     * last mouse event.
+     */
+
+    if (eventWindow == NULL) {
+	eventWindow == _windowWithMouse;
+    }
+    if (eventWindow) {
+
+	/*
+	 * Set the local mouse position to its NSWindow flipped coordinates,
+	 * with the origin at top left, and the global mouse position to the
+	 * flipped screen coordinates.
+	 */
+
 	global = [eventWindow tkConvertPointToScreen: local];
 	local.y = [eventWindow frame].size.height - local.y;
 	global.y = tkMacOSXZeroScreenHeight - global.y;
-    } else { /* local will be in screen coordinates. */
-	if (_windowWithMouse ) {
-	    eventWindow = _windowWithMouse;
-	    global = local;
-	    local = [eventWindow tkConvertPointFromScreen: local];
-	    local.y = [eventWindow frame].size.height - local.y;
-	    global.y = tkMacOSXZeroScreenHeight - global.y;
-	} else { /* We have no window. Use the screen???*/
-	    local.y = tkMacOSXZeroScreenHeight - local.y;
-	    global = local;
-	}
-    }
-
-    TkWindow *winPtr = TkMacOSXGetTkWindow(eventWindow);
-    Tk_Window tkwin = (Tk_Window) winPtr;
-
-    if (tkwin) {
-	TkWindow *grabWinPtr = winPtr->dispPtr->grabWinPtr;
-	if (grabWinPtr &&
-	    grabWinPtr != winPtr &&
-	    !winPtr->dispPtr->grabFlags && /* this means the grab is local. */
-	    grabWinPtr->mainPtr == winPtr->mainPtr) {
-	    return theEvent;
-	}
+
+    } else {
+
+	/*
+	 * As a last resort, with no NSWindow to work with, set both local and
+	 * global to the screen coordinates.
+	 */
+
+	local.y = tkMacOSXZeroScreenHeight - local.y;
+	global = local;
+    }
+
+    /*
+     * Find the toplevel which corresponds to the event NSWindow.
+     */
+
+    winPtr = TkMacOSXGetTkWindow(eventWindow);
+    if (winPtr == NULL) {
+	tkwin = TkMacOSXGetCapture();
+	winPtr = (TkWindow *)tkwin;
     } else {
-	tkwin = TkMacOSXGetCapture();
+	tkwin = (Tk_Window) winPtr;
     }
     if (!tkwin) {
 	TkMacOSXDbgMsg("tkwin == NULL");
 	return theEvent; /* Give up.  No window for this event. */
-    } else {
-	winPtr = (TkWindow *)tkwin;
+    }
+
+    /*
+     * If another toplevel has a grab, we ignore the event.
+     */
+
+    grabWinPtr = winPtr->dispPtr->grabWinPtr;
+    if (grabWinPtr &&
+	grabWinPtr != winPtr &&
+	!winPtr->dispPtr->grabFlags && /* this means the grab is local. */
+	grabWinPtr->mainPtr == winPtr->mainPtr) {
+	return theEvent;
     }
 
+    /*
+     * Convert local from NSWindow flipped coordinates to the toplevel's
+     * coordinates.
+     */
+
     local.x -= winPtr->wmInfoPtr->xInParent;
     local.y -= winPtr->wmInfoPtr->yInParent;
 
+    /*
+     * Find the containing Tk window, and convert local into the coordinates
+     * of the Tk window.  (The converted local coordinates are only needed
+     * for scrollwheel events.)
+     */
+
     int win_x, win_y;
     tkwin = Tk_TopCoordsToWindow(tkwin, local.x, local.y, &win_x, &win_y);
+    local.x = win_x;
+    local.y = win_y;
+
+    /*
+     *  Generate an XEvent for this mouse event.
+     */
 
     unsigned int state = 0;
     NSInteger button = [theEvent buttonNumber];
     EventRef eventRef = (EventRef)[theEvent eventRef];
     UInt32 buttons;
@@ -180,15 +225,25 @@
     if (modifiers & NSFunctionKeyMask) {
 	state |= Mod4Mask;
     }
 
     if (eventType != NSScrollWheel) {
+
+	/*
+	 * For normal mouse events, Tk_UpdatePointer will send the XEvent.
+	 */
+
 #ifdef TK_MAC_DEBUG_EVENTS
 	TKLog(@"UpdatePointer %p x %f.0 y %f.0 %d", tkwin, global.x, global.y, state);
 #endif
 	Tk_UpdatePointer(tkwin, global.x, global.y, state);
-    } else { /* handle scroll wheel event */
+    } else {
+
+	/*
+	 * For scroll wheel events we need to send the XEvent here.
+	 */
+	
 	CGFloat delta;
 	int coarseDelta;
 	XEvent xEvent;
 
 	xEvent.type = MouseWheelEvent;