You can replicate the issue using the fiddle at http://zkfiddle.org/sample/liqn2c/6-Bad-window-dragging-behaviour
If you create a popup window and you move it around the screen, you can get two different bad behaviours depending on which you use between Firefox or Chrome.
Open a popup window and then move it out of the browser window on the left side.
You won't be able to get the window anymore and, since all other objects are grayed out, you have to refresh the page.
Open a popup window and then move it out of the browser window on the bottom side.
A scroll bar will appear on the right side of the browser: scroll down to show the window you just moved.
Try to move up the window again: you will see that it will be suddenly moved to another position not aligned with mouse cursor.