Widgets: Prevent currently active wide widget controls from being partially...

Widgets: Prevent currently active wide widget controls from being partially covered by another widget's controls.

The currently active widget controls should always be displayed on top.

Props audrasjb, fervillz, rinkuyadav999, jaydeep23290, ashokrd2013, melchoyce, pento, westonruter, SergeyBiryukov.
Fixes #42001.
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git-svn-id: https://core.svn.wordpress.org/trunk@47063 1a063a9b-81f0-0310-95a4-ce76da25c4cd