Dialog broken when reaches the end of the page

[blaatunga]

When the dialog resized out of the viewport of browser, the buttons disappear.

Example to reproduce the bug:

• ​http://jqueryui.com/themeroller/ - in the examples click Open Dialog
• scroll down to the bottom of the page and drag there the dialog
• it won't let you drag it under the page end, but:
• if you now resize it, you can go under the window end with the buttons on the dialog
• when you drag the dialog up, the buttons will be missing. Resizing then has no effect on this.

Tested on IE, FF, Safari, Chrome on Win7

[Scott González]

Duplicate of #4603.