Explanation of this part of code in mbdesktop.c

I have some problems with selection of the proper active_item in the
desktop window.

Anyone can brief me what the handle_button_event(MBDesktop *mb,
XButtonEvent *e) API does in matchbox-desktop/mbdesktop.c

Please tell me what you feel...thank you

void
handle_button_event(MBDesktop *mb, XButtonEvent *e)
{
  XEvent ev;
  MBDesktopItem *active_item = NULL;

  active_item = mbdesktop_item_get_from_coords(mb, e->x, e->y); 
  
  if (active_item)
    {
      Bool canceled = False;
           
      if (XGrabPointer(mb->dpy, mb->win_top_level, False,
		       ButtonPressMask|ButtonReleaseMask|		      
PointerMotionMask|EnterWindowMask|LeaveWindowMask,
		       GrabModeAsync,
		       GrabModeAsync, None, None, CurrentTime)
	  == GrabSuccess)
	{
      	
	  if (mb->have_focus && mb->had_kbd_input)
	    {
	      mbdesktop_view_item_highlight (mb, mb->kbd_focus_item, 					   
 HIGHLIGHT_OUTLINE_CLEAR);
                     }

	  mbdesktop_view_item_highlight (mb, active_item, HIGHLIGHT_FILL); 
    
	  /// Action starts here for next mouse input.....So causes
error.....When swapped with keyboard action starts at if(active_item)
causing correct behavior
		  
	  for (;;) 
	    {
	      int x1 = active_item->x;
	      int x2 = active_item->x + active_item->width;
	      int y1 = active_item->y;
	      int y2 = active_item->y + active_item->height;

	    XMaskEvent(mb->dpy,
			 ButtonPressMask|ButtonReleaseMask|
			 PointerMotionMask, &ev);
    
	      switch (ev.type)
		{
		case MotionNotify:
     
		  if (canceled 
		      && ev.xmotion.x > x1
		      && ev.xmotion.x < x2
		      && ev.xmotion.y > y1
		      && ev.xmotion.y < y2
		      )
		    {
		      mbdesktop_view_item_highlight (mb, active_item, HIGHLIGHT_FILL); 
		      canceled = False;
		      break;
		    }
		  if (!canceled 
		      && (    ev.xmotion.x < x1
			   || ev.xmotion.x > x2
			   || ev.xmotion.y < y1
			   || ev.xmotion.y > y2
			   ))
		  {
		    mbdesktop_view_item_highlight (mb, active_item, HIGHLIGHT_FILL_CLEAR); 
		    canceled = True;
		    break;
		  }

		  break;
		case ButtonRelease:
     
		  XUngrabPointer(mb->dpy, CurrentTime);
		  if (!canceled)
		    {
		      mbdesktop_view_item_highlight (mb, active_item, 
						     HIGHLIGHT_FILL_CLEAR); 
		      if (active_item->activate_cb)
			active_item->activate_cb((void *)mb, 
						 (void *)active_item); 
      	
		      return;
			}
		  else
		    {
		      if (mb->have_focus && mb->had_kbd_input)
			{
			  mb->kbd_focus_item = active_item;
			  mbdesktop_view_item_highlight (mb, 
							 mb->kbd_focus_item, 
							 HIGHLIGHT_OUTLINE_CLEAR); 
			}

      	
		      return;
		    }
		}
	    }
	}

    }
  else
    {
      if (e->y < mb->workarea_y + 20)
	{
	  if (e->x > (mb->workarea_x + mb->workarea_width -24))
	    mbdesktop_scroll_up(mb);
	  else if (e->x > (mb->workarea_x + mb->workarea_width -40))
	    mbdesktop_scroll_down(mb);

	  mbdesktop_view_paint(mb, False);
      	
	}
    }
}

Thanks