gistfile1.js

    findScrollable: function findScrollable(dir, horizontal) {
        let pos = "scrollTop", size = "clientHeight", max = "scrollHeight", layoutSize = "offsetHeight",
            overflow = "overflowX", border1 = "borderTopWidth", border2 = "borderBottomWidth";
        if (horizontal)
            pos = "scrollLeft", size = "clientWidth", max = "scrollWidth", layoutSize = "offsetWidth",
            overflow = "overflowX", border1 = "borderLeftWidth", border2 = "borderRightWidth";

        function find(elem) {
            for (; elem && elem.ownerDocument; elem = elem.parentNode) {
                let style = util.computedStyle(elem);
                let realSize = elem[size];
                // Stupid Gecko eccentricities. May fail for quirks mode documents.
                if (style.overflow === "visible") // Stupid, fallible heuristic.
                    continue;
                if (dir < 0 && elem[pos] > 0 || dir > 0 && elem[pos] + realSize < elem[max])
                    break;
            }
            return elem;
        }

        let win = this.focusedWindow;
        if (win.getSelection().rangeCount)
            var elem = find(win.getSelection().getRangeAt(0).startContainer);
        if (!(elem instanceof Element)) {
            let doc = Buffer.findScrollableWindow().document;
            elem = (doc.compatMode === "CSS1Compat" ? doc.documentElement : doc.body) || doc.getElementsByTagName("body")[0];
        }
        return elem;
    },