Selectmenu items remains visible and fixed while scrolling dialog content

[mcanepa]

Inside a dialog when start scrolling it's content, if a selectmenu was opened, the items container shows like a fixed element.

Steps:

1. Click on select menu to show items
2. Using the mousewheel, start scrolling down

Result: Items remain visible, fixed and on top of the rest of the content

Expected: Maybe the items must hide or move along with the select menu.

The same happens with the datepicker widget

Demo: ​http://jsfiddle.net/w8bqpmev/