Highlight the whole Widget Area when it is selected

[mapk]

Problem

When clicking a collapsed widget area, the area's heading is highlighted with a blue border. This is good. However when the area is opened, the heading remains highlighted instead of the whole area. This doesn't look right.

Example of problem

Solution

Let's make the whole widget area highlighted in a blue border when it is selected, not just the area heading.

[talldan]

I closed a similar issue related to this. I don't think this is the right thing to do, because it'd involve removing a focus style from the expand/collapse button, which is basically an accessibility regression.

None of the other accordions in WordPress do this, so I'm not sure why the widget area should be different.