Фиксируем плавающую панель сайта и панель админки Битрикса (Fixed Bitrix panel, bx-panel)

script.js

/*
https://gist.github.com/lekomtsev/d7fe3bfad8f42307fa2149e0816f3740
 */
var winWidth = function() {

	var w = 0;

	// IE
	if (typeof( window.innerWidth ) != 'number') {

		if (!(document.documentElement.clientWidth === 0)) {

			// strict mode
			w = document.documentElement.clientWidth;
		} else {

			// quirks mode
			w = document.body.clientWidth;
		}
	} else {

		// w3c
		w = window.innerWidth;
	}

	return w;
};

/*
https://gist.github.com/lekomtsev/d7fe3bfad8f42307fa2149e0816f3740
 */
if(!!BX) {
	BX.ready(function () {
		if (BX.admin){
			var arMyPanel = document.querySelectorAll(".header__info,.popup-history-description");
			var	BxPanel = BX.admin.panel,
				FxPanel = function () {
					let winWidthVal = winWidth();
					arMyPanel.forEach(function (currentValue, currentIndex, listObj) {
						let MyPanel = currentValue;
						if(winWidthVal<768){
							if (window.pageYOffset >= BxPanel.DIV.clientHeight && BxPanel.isFixed() === false) {
								MyPanel.style.top = 0;
							} else if (BxPanel.isFixed() === true) {
								MyPanel.style.top = BxPanel.DIV.clientHeight + "px";
							} else {
								MyPanel.style.top = BxPanel.DIV.clientHeight - window.pageYOffset + "px";
							}
						}else{
							MyPanel.style.top = 0;
						}
					});
				};

			if (!!arMyPanel && arMyPanel.length>0) {
				FxPanel();
				window.onresize = FxPanel;
				window.onscroll = FxPanel;
				BX.addCustomEvent('onTopPanelCollapse', BX.delegate(FxPanel, this));
				BX.addCustomEvent('onTopPanelFix', BX.delegate(FxPanel, this));
			}
		}
	});
}