Find which DOM item makes your container go too big.

ftf.js

/*vim: fileencoding=utf8 tw=100 expandtab ts=4 sw=4 */
/*jslint indent: 4, maxlen: 100, browser: true */
/*globals console, Element*/

(function (exports) {
    'use strict';

    function findTheFucker(root, width, restoreAll) {
        var fucker;

        function isFucked() {
            return root.getBoundingClientRect().width > width;
        }

        function plantCss() {
            var style = document.getElementById('ftf-style');

            if (style) {
                return;
            }

            style = document.createElement('style');
            style.type = 'text/css';
            style.innerHTML = '.--ftf { display: none; position: absolute; }';
            style.id = 'ftf-style';

            document.querySelector('head').appendChild(style);
        }

        function hideItAll(node) {
            if (!(node instanceof Element)) {
                return;
            }

            node.classList.add('--ftf');
            Array.prototype.forEach.call(node.childNodes, hideItAll);
        }

        function showItAll(node) {
            if (!(node instanceof Element) || (fucker && !restoreAll)) {
                return;
            }

            node.classList.remove('--ftf');

            if (!fucker && isFucked()) {
                fucker = node;
                return;
            }

            Array.prototype.forEach.call(node.childNodes, showItAll);
        }

        (function () {
            console.log('Who is fucking your ' + width + 'px width?', root);

            if (!isFucked()) {
                return console.log('Wait. It is not fucked actually.');
            }

            plantCss();
            hideItAll(root);
            showItAll(root);

            if (fucker) {
                console.log('Fucker found!');
                console.log(fucker);
            } else {
                console.log('No fucker found. Did you set the width manually?');
            }
        }());
    }

    exports.ftf = findTheFucker;
}(window));