IE9 doesn't understands `vh ` (vertical height) when inside of an `iframe`. Normally IE9 will understand `vh` as the vertical height relative to the viewport, but in an `iframe`, IE9 thinks the vertical height is relative to the size of the whole page. So this fixes that when the site is viewed in the Customizer preview – an `iframe`.
Props laurelfulford.
Fixes#38722.
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David A. Kennedy