Misplaced user picker in the "share by email" dialog if page is scrolled

How to reproduce:

• open a page which is long (browser displays a scroll) - e.g. Sandbox.WebHome
• click on the share page by email button
• scroll the page under the dialog box without closing the dialog (see 1-scroll.png)
• type a user name in the user picker. Make sure it's a name that matches an existing user
• a loading spinner will display in the input box showing that suggestions are loading, at some point it will dissapear but no suggestions will be displayed (see 2-noSuggestions.png)
• scroll back up to the top of the page (without closing the dialog), the suggestions are actually displayed on the top of the screen, and if you scroll up to the very top of the page the suggestions will be correctly positioned under the input (see 3-suggestionsOnTop.png and 4-suggestionsFine.png)
• close the dialog by clicking on the x button on the top right, while the suggestions list is still displayed. The suggestions will stay on the screen (see 5-leftoverSuggestions.png)
  Additional usecase:
• reload the page, click again share page by mail, this time without scrolling the page
• type the name of an existing user in the user picker input
• the suggestions will display fine
• while the suggestions list is still displayed, scroll the page under the dialog without closing the dialog (position your mouse outside the dialog and scroll)
• the suggestions will follow the scroll (scroll out of the screen) while the share page by email dialog will remain fixed on top of the visible window.

Expected result:

• the suggest box should always be visible in the share page by email dialog, regardless of how much scroll is done and when it's done.

Please note that this problem becomes more important if a customization of the skin is made which positions the "share page by email" button lower in the page and a scrolling of the page is indispensable in order to be able to click the button. In this case, if the user does not scroll the page back up after having displayed the dialog, they will never see the suggestions properly, as the suggestions will always be placed relative to the top of the scrolled page, not to the share by email dialog.