load-recent-posts-upon-panel-expansion.js

// Watch for the first expansion of each added panel.
var logPanelFirstExpanded = function( panel ) {
	var onceExpanded, onExpandedChange;

	if ( ! panel.extended( wp.customize.Posts.PostsPanel ) ) {
		return;
	}

	onceExpanded = function() {
		console.info( 'Panel "%s" expanded! You should now lazy-load your controls if needed.', panel.id );
	};
	if ( panel.expanded.get() ) {
		onceExpanded( panel ); // Already expanded so no need for onExpandedChange.
	} else {
		onExpandedChange = function( isExpanded ) {
			if ( isExpanded ) {
				panel.expanded.unbind( onExpandedChange ); // Value#unbind().
				onceExpanded( panel );
			}
		};
		panel.expanded.bind( onExpandedChange ); // Value#bind().
	}
};

// Watch any panels already added.
wp.customize.panel.each( logPanelFirstExpanded );

// Watch for panels newly added.
wp.customize.panel.bind( 'add', logPanelFirstExpanded ); // Events#bind().