Nav Menus quick search spinner position

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Some (old) operating systems have scrollbars that take space and don't use "overlay scrollbars". When searching items in the nav menus screen, XP, Vista and Windows 7 users will see a misplaced spinner because scrollbars take space. See screenshot below, from left to right: latest Chrome, Firefox and IE 8

When the spinner disappears, it will still affect layout and that big empty space below the input field will still be there.

I guess this doesn't happen on Windows 8, modern Linux distros, Macs, iOS, Android, etc. since they all use "overlay scrollbars".

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After the patch:

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To reproduce (Windows XP, Vista, 7):

• do a first search and be sure to get enough results to make the scrollbar appear
• notice the empty space below the search field
• do a second search
• notice the spinner displayed under the search field

[afercia]

In 34065:

Nav Menus: fix the quick search spinner position on Operating Systems that show scrollbars.

Fixes #33290.