Empty widget areas get deactivated in the Customizer

[celloexpressions]

There's a regression here, likely related to #28709, been confirmed by several people in slack. Makes it impossible to add new widget to an empty sidebar in the Customizer - once it's emptied it becomes inactive (hiding).

[westonruter]

Here is a shortcut which can be pasted into the browser console to empty out all sidebars to trigger the bad behavior:

_.each( wp.customize.panel('widgets').sections(), function ( section ) {
    var controls = section.controls(); 
    controls.pop().setting( [] ); // the last control is the SidebarWidgets control, the list of widget IDs in the sidebar
} );

[westonruter]

​https://github.com/xwp/wordpress-develop/pull/55

[westonruter]

The problem seems to be the SidebarSection.isContextuallyActive override of Section.isContextuallyActive, which was introduced in #30235, specifically via commit ​c91db4d. This method override actually was taking things too far. Now that that SidebarSection's active state is getting sent from the Preview, we can just rely on it to show/hide the section. We don't want to hide a section if all of the widgets in a section are removed or if they are all inactive (e.g. hidden via Jetpack Widget Visibility). So the method override I added was just plain wrong, I believe.

In 30378.diff​ I remove this method override.

[ocean90]

In 30552:

Customizer: Don't override Section.isContextuallyActive() in SidebarSection.

This fixes a bug where empty widget areas get deactivated in the Customizer.

fixes #30378.
see #30235.
props westonruter.