Fix search box position on the Add Plugins screen after resizing the window to narrower than 1000px and then back to wider than 1200px.

Merges [30830] to the 4.1 branch.

Fixes #30658.

Built from https://develop.svn.wordpress.org/branches/4.1@30878


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30866 1a063a9b-81f0-0310-95a4-ce76da25c4cd