Calculate the root h/w every time in case it changes

src/nimwin.nim

@@ -95,12 +95,11 @@ when isMainModule:
   start.subWindow = None
 
   discard XSetErrorHandler(ignoreBadWindows)
-  discard display.XGetWindowAttributes(root, attr.addr)
 
+  while true:
+    discard display.XGetWindowAttributes(root, attr.addr)
     let rootWidth = attr.width
     let rootHeight = attr.height
-
-  while true:
     sleep(10)
 
     for window in getChildren(display, rootHeight, rootWidth):