Limit number of Widgets in Sidebars

I solved this in Javascript. If you want to completely prevent it you should also do it server-side, because you can edit the widgets with Javascript disabled (try it out!).

The different sidebars are checked when you drop widgets to them or away from them. If they become full, the background color changes and you can no longer drop items on them. If, on startup, the sidebar is already more than full (because you tightened the restriction), the background color becomes red. You can still drag widgets away from full widgets to make them empty again.

Please test out this code, to find ways to add or remove widgets that I missed. The “magic” in the jQuery code comes from Aman, who answered a Stack Overflow question I posted about it.

Javascript:

jQuery( function( $ ) {
    var sidebarLimits = {
        'sidebar-1': 2,
        'sidebar-2': 2,
    };
    var realSidebars = $( '#widgets-right div.widgets-sortables' );
    var availableWidgets = $( '#widget-list' ).children( '.widget' );

    var checkLength = function( sidebar, delta ) {
        var sidebarId = sidebar.id;
        if ( undefined === sidebarLimits[sidebarId] ) {
            return;
        }

        // This is a limited sidebar
        // Find out how many widgets it already has
        var widgets = $( sidebar ).sortable( 'toArray' );
        $( sidebar ).toggleClass( 'sidebar-full', sidebarLimits[sidebarId] <= widgets.length + (delta || 0) );
        $( sidebar ).toggleClass( 'sidebar-morethanfull', sidebarLimits[sidebarId] < widgets.length + (delta || 0) );

        var notFullSidebars = $( 'div.widgets-sortables' ).not( '.sidebar-full' );
        availableWidgets.draggable( 'option', 'connectToSortable', notFullSidebars );
        realSidebars.sortable( 'option', 'connectWith', notFullSidebars );
    }

    // Check existing sidebars on startup
    realSidebars.map( function() {
        checkLength( this );
    } );

    // Update when dragging to this (sort-receive)
    // and away to another sortable (sort-remove)
    realSidebars.bind( 'sortreceive sortremove', function( event, ui ) {
        checkLength( this );
    } );

    // Update when dragging back to the "Available widgets" stack
    realSidebars.bind( 'sortstop', function( event, ui ) {
        if ( ui.item.hasClass( 'deleting' ) ) {
            checkLength( this, -1 );
        }
    } );

    // Update when the "Delete" link is clicked
    $( 'a.widget-control-remove' ).live( 'click', function() {
        checkLength( $( this ).closest( 'div.widgets-sortables' )[0], -1 );
    } );
} );

CSS:

.sidebar-full
{
    background-color: #cfe1ef !important;
}

.sidebar-morethanfull
{
    background-color: #c43 !important;
}

PHP to load them:

$wpse19907_file = $plugin;
add_action( 'admin_enqueue_scripts', 'wpse19907_admin_enqueue_scripts' );
function wpse19907_admin_enqueue_scripts( $hook_suffix )
{
    if ( 'widgets.php' == $hook_suffix ) {
        wp_enqueue_script( 'wpse-19907', plugins_url( 'wpse-19907.js', $GLOBALS['wpse19907_file'] ), array(), false, true );
        wp_enqueue_style( 'wpse-19907', plugins_url( 'wpse-19907.css', $GLOBALS['wpse19907_file'] ) );
    }
}

An attempt at a server-side check (probably not complete yet):

$wpse19907_sidebars_max_widgets = array(
    'sidebar-1' => 2,
);

add_action( 'sidebar_admin_setup', 'wpse19907_sidebar_admin_setup' );
function wpse19907_sidebar_admin_setup()
{
    if ( ! isset( $_POST['action'] ) || 'save-widget' != $_POST['action'] || empty( $_POST['add_new'] ) ) {
        return;
    }

    // We're adding a new widget to a sidebar
    global $wpse19907_sidebars_max_widgets;
    $sidebar_id = $_POST['sidebar'];

    if ( ! array_key_exists( $sidebar_id, $wpse19907_sidebars_max_widgets ) ) {
        return;
    }

    $sidebar = wp_get_sidebars_widgets();
    $sidebar = isset( $sidebars[$sidebar_id] ) ? $sidebars[$sidebar_id] : array();

    if ( count( $sidebar ) <= $wpse19907_sidebars_max_widgets[$sidebar_id] ) {
        die( 'mx' ); // Length must be shorter than 2, and unique
    }
}