We've discovered a memory leak in basewindow. Windows create a windowlist on-the-fly to keep track of which window should be frontmost. That means that every time a window is created there's a reference created (which would stop it being garbage-collected).

Here's the bit of code:
        <method name="init">
            // Windows that are datamappped control their own visibility
            if (this.datapath != null) {
                this.datapath.setAttribute('datacontrolsvisibility',
                    this.visibility == "collapse");
            }

            super.init();
            this.mousedown_del = new LzDelegate ( this, "_mousedown", this , "onmousedown" );
            this.mouseup_del = new LzDelegate ( this, "_mouseup", this , "onmouseup" );

            if (this.visible) {
                var wlist = parent.options['windowlist'];
                wlist.push(this); // frontmost window for now
                this.setAttribute('haswindowfocus', true);
            }
        </method>

The destroy method should clear out the references in windowlist:

        <!--- @keywords private -->
        <method name="destroy">
          <!-- TODO: [2005-04-07 ptw] remove when cleanup is automatic -->
          if (this.mousedel) {
              this.mousedel.unregisterAll();
          }
          super.destroy();
        </method>