🚀 go-pugleaf

A basic (I hope) PM question:

I have an analog clock application that experiences a clipping peculiarity
when it is partly overlayed by a dialog or a pull down menu.  eg:

	+-------+
	|\   	| (1)
  (2) +======+	|
      +======+	|
	|   \	|
	+-------+

(1) is the main window, (2) is a dialog that is partly overlaying the
window, the "\" symbols represent a line that is partly overlayed by
the dialog.  The problem is that often the drawing that takes place outside
of the overlayed area (outside not under it) is incorrect.  eg:      

	1) draw a line
	2) overlay window with a dialog, partly covering the line
	3) erase the line by redrawing it using the background color
	4) remove the dialog 

At the end of this operation I end up with the line partly erased, the
portion under the dialog is correct but that portion not covered by the
dialog is not.  eg:

	+-------+
	|\   	| 
  	|	|
      	|	|
	|   \	|
	+-------+

Can someone explain why this is happening and what can be done to fix it?