User:HyperNervie/sandbox

Input these in your browser console:

mw.loader.using("oojs-ui-widgets").then(function () {
    function convertToOOUITabber(_, tabberDiv) {
        const $tabberDiv = $(tabberDiv);
        const $labels = $tabberDiv.find(
            "> .rhwiki-tabberneue-menu" +
            "> .rhwiki-tabberneue-option" +
            "> .rhwiki-tabberneue-label"
        );
        const $panels = $tabberDiv.find(
            "> .rhwiki-tabberneue-content" +
            "> .rhwiki-tabberneue-panel"
        );
        const tabPanelLayouts = [];

        $labels.each(function (i, labelDiv) {
            tabPanelLayouts.push(new OO.ui.TabPanelLayout({
                name: i,
                expanded: false,
                label: $(labelDiv).contents(),
                $content: $($panels[i]).contents()
            }));
        });

        const classList = tabberDiv.classList;
        const indexLayout = new OO.ui.IndexLayout({
            expanded: false,
            autoFocus: false
        });
        let shownIndex = parseInt($tabberDiv.attr("data-shown-index"));
        if (isNaN(shownIndex) || shownIndex < 0 || shownIndex >= $panels.length)
            shownIndex = 0;
        indexLayout.addTabPanels(tabPanelLayouts).setTabPanel(shownIndex);

        $tabberDiv.replaceWith(indexLayout.$element);
    }

    mw.hook("wikipage.content").add(function ($content) {
        while (true) {
            let $tabbers = $content.find(".rhwiki-tabberneue");
            if ($tabbers.length === 0) break;
            $tabbers.each(convertToOOUITabber);
        }
    });
});

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Current "TabStart-content-TabEnd" solution renders a series of pairs of label and panel. Before MediaWiki:Gadget-Tabber.js is loaded, all panels are visible. In extreme cases where the script fails to load, the panels are left there forever.

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Here we have only one visible panel at the beginning, and the labels are styled properly so that when MediaWiki:Gadget-Tabber.js loads, there won't be any change to page appearance. Even if the script doesn't load successfully, readers just can't switch panels by clicking on labels.