I have an app that allows multiple windows. I have a strange bug where, if I repeatedly open and close windows, new windows are positioned lower and lower on the screen. I would expect this if I were keeping the windows open, but it seems that the OS X window tiling mechanism is unaware of when my windows are closing.

Potentially relevant notes: I am using garbage collection. This is not a document-based app. When I close a window, the corresponding menu item is removed from the "Window" menu.

Any hints as to what I might be doing wrong would be appreciated. Thanks!