docs/headers.js

for (const el of document.querySelectorAll("[id]")) {
    el.classList.add("haspara");
    const pilcrow = document.createElement("a");
    pilcrow.className = "pilcrow";
    pilcrow.href = "#" + el.id;
    pilcrow.innerHTML = "¶";
    el.prepend(pilcrow);
}

const foldable = (root, children) => {
    let state = true;
    root.addEventListener("click", (e) => {
        if (e.target.classList.contains("pilcrow")) state = true;
        else state = !state;

        children.style.height = state ? "auto" : "0";
        children.style.overflow = state ? "visible" : "hidden";
        if (state) root.classList.remove("closed");
        else root.classList.add("closed");
    });
    root.classList.add("toggle-root");
};

const make_foldable = (root) => {
    const child_stacks = new Array(10).fill(null).map(() => ({ children: [], root: null }));

    const flush_header = (this_level, sibling) => {
        for (let level = 9; level >= this_level; level--) {
            const stack = child_stacks[level];

            if (!stack.root) continue;

            const new_e = document.createElement("div");
            for (const old_e of stack.children) {
                old_e.remove();
                new_e.appendChild(old_e);
            }

            foldable(stack.root, new_e);
            let parent_level;
            for (parent_level = level - 1; parent_level > 0; parent_level--) if (child_stacks[parent_level].root) break;

            if (parent_level === -1) {
                if (sibling) root.insertBefore(new_e, sibling);
                else root.appendChild(new_e);
            } else {
                stack.root.remove();
                child_stacks[parent_level].children.push(stack.root);
                child_stacks[parent_level].children.push(new_e);
            }

            stack.root = null;
            stack.children.length = 0;
        }
    };

    for (const child of [...root.children]) {
        if (/^H\d$/.test(child.tagName)) {
            const this_level = parseInt(child.tagName[1]) - 1;

            flush_header(this_level, child);

            child_stacks[this_level].root = child;
            continue;
        }

        for (let level = 9; level >= 0; level--) {
            if (child_stacks[level].root) {
                child_stacks[level].children.push(child);
                break;
            }
        }
    }

    for (let level = 9; level >= 0; level--) {
        flush_header(level, null);
    }
};
make_foldable(document.body);