Window constrain problem

Hi!

I've discovered following problem: I have a tab panel. I want render a Window to one of these tab's. On window I set the constrain to true. It's working well, but when I want move my window to top, then I get a "tabbar-height" distance between my window and my tabbar. Maybe I was not clear, please check my example:

http://ift.tt/1Gpu4wF

Click to "Create Window" button. After try move the Window over the tabbar. Here is a picture of my problem:

window.png

Window constrain problem