There is a bug in dell display manager (DDM). The Easy Arrange functionality doesn't seem to work if you are using font & element scaling in Windows. The windows snap to wrong positions. In my windows 7 I have set font & element scaling in display > make text and other items larger or smaller. If I use 100%, it works. I think this applies to all windows versions. I hope you can fix this.
win7, ddm 1.586
I could be wrong but I think you'll find that the issue only arises under Win7 with older apps that are *not* DPI aware. Do you see the same problem with native Win7 DPI-aware apps like IE or WordPad or Paint?
Luckily I was able to fix this problem by myself. I had to change the DPI to 100% (+relogin) and then back to 140 (+relogin) to get this to work. After doing this it's working fine. However, I think you should have a look of this as it doesn't work right away. Try to set DPI e.g. to 150% and then try to install DDM in win7. It was the same thing with all apps.