Fix selectivity dropdowns when scrolling

When a nuxeo-selectivity dropdown is opened and user scrolls the page, the dropdown does not stick with the corresponding selection.

Steps to Reproduce:

1. Create a document
2. Start typing in the tags dropdown
3. Scroll down the page

Results: The dropdown doesn't stick with the tag dropdown and continues to show on the page as you scroll down.

Also reproducible doing the following:

1. Go to any folder and click on Add Content (plus) button.
2. Click on Import tab and add file
3. Click on Add Properties Button
4. Select an asset Type
5. Click one of the properties dropdown menus and try to scroll outside this dropdown