• The easiest way would be in overwriting the template threadPostList.

    Add the following before line 205 ({event name='messageFooterButtons'})

    <li><a href="#top" title="{lang}wcf.global.scrollUp{/lang}" class="button jsTooltip jumpToTop"><span class="icon icon16 fa-arrow-up"></span></a></li>

    And the following at the end of the template:

    <script data-relocate="true">
    require(['WoltLabSuite/Core/Ui/Page/JumpToTop'], function(UiPageJumpToTop) {
    var buttons = elBySelAll('.jumpToTop');
    for (var i = 0; i < buttons.length; i++) {
    buttons[i].addEventListener(WCF_CLICK_EVENT, UiPageJumpToTop.prototype._jump.bind(this));