Hugo Sidebar Table of Contents

The PaperMod theme places the article’s table of contents at the top by default, which is not convenient for jumping around while reading. So, referring to Sulv’s settings, I modified it into a sidebar TOC.

Steps

1. Modify toc.html

In a previous article I mentioned that modifications made at the site root have higher priority and override the theme’s original content. So use the following commands to generate and edit toc.html:

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mkdir layouts/partials -p \
  && nano layouts/partials/toc.html

Paste the following content:

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{{- $headers := findRE "<h[1-6].*?>(.|\n])+?</h[1-6]>" .Content -}}
{{- $has_headers := ge (len $headers) 1 -}}
{{- if $has_headers -}}
<aside id="toc-container" class="toc-container wide">
    <div class="toc">
        <details {{if (.Param "TocOpen") }} open{{ end }}>
            <summary accesskey="c" title="(Alt + C)">
                <span class="details">{{- i18n "toc" | default "Table of Contents" }}</span>
            </summary>

            <div class="inner">
                {{- $largest := 6 -}}
                {{- range $headers -}}
                {{- $headerLevel := index (findRE "[1-6]" . 1) 0 -}}
                {{- $headerLevel := len (seq $headerLevel) -}}
                {{- if lt $headerLevel $largest -}}
                {{- $largest = $headerLevel -}}
                {{- end -}}
                {{- end -}}

                {{- $firstHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers 0) 1) 0)) -}}

                {{- $.Scratch.Set "bareul" slice -}}
                <ul>
                    {{- range seq (sub $firstHeaderLevel $largest) -}}
                    <ul>
                        {{- $.Scratch.Add "bareul" (sub (add $largest .) 1) -}}
                        {{- end -}}
                        {{- range $i, $header := $headers -}}
                        {{- $headerLevel := index (findRE "[1-6]" . 1) 0 -}}
                        {{- $headerLevel := len (seq $headerLevel) -}}

                        {{/* get id="xyz" */}}
                        {{- $id := index (findRE "(id=\"(.*?)\")" $header 9) 0 }}

                        {{- /* strip id="" to leave xyz, no way to get regex capturing groups in hugo */ -}}
                        {{- $cleanedID := replace (replace $id "id=\"" "") "\"" "" }}
                        {{- $header := replaceRE "<h[1-6].*?>((.|\n])+?)</h[1-6]>" "$1" $header -}}

                        {{- if ne $i 0 -}}
                        {{- $prevHeaderLevel := index (findRE "[1-6]" (index $headers (sub $i 1)) 1) 0 -}}
                        {{- $prevHeaderLevel := len (seq $prevHeaderLevel) -}}
                        {{- if gt $headerLevel $prevHeaderLevel -}}
                        {{- range seq $prevHeaderLevel (sub $headerLevel 1) -}}
                        <ul>
                            {{/* the first should not be recorded */}}
                            {{- if ne $prevHeaderLevel . -}}
                            {{- $.Scratch.Add "bareul" . -}}
                            {{- end -}}
                            {{- end -}}
                            {{- else -}}
                            </li>
                            {{- if lt $headerLevel $prevHeaderLevel -}}
                            {{- range seq (sub $prevHeaderLevel 1) -1 $headerLevel -}}
                            {{- if in ($.Scratch.Get "bareul") . -}}
                        </ul>
                        {{/* manually do pop item */}}
                        {{- $tmp := $.Scratch.Get "bareul" -}}
                        {{- $.Scratch.Delete "bareul" -}}
                        {{- $.Scratch.Set "bareul" slice}}
                        {{- range seq (sub (len $tmp) 1) -}}
                        {{- $.Scratch.Add "bareul" (index $tmp (sub . 1)) -}}
                        {{- end -}}
                        {{- else -}}
                    </ul>
                    </li>
                    {{- end -}}
                    {{- end -}}
                    {{- end -}}
                    {{- end }}
                    <li>
                        <a href="#{{- $cleanedID -}}" aria-label="{{- $header | plainify -}}">{{- $header | safeHTML -}}</a>
                        {{- else }}
                    <li>
                        <a href="#{{- $cleanedID -}}" aria-label="{{- $header | plainify -}}">{{- $header | safeHTML -}}</a>
                        {{- end -}}
                        {{- end -}}
                        <!-- {{- $firstHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers 0) 1) 0)) -}} -->
                        {{- $firstHeaderLevel := $largest }}
                        {{- $lastHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers (sub (len $headers) 1)) 1) 0)) }}
                    </li>
                    {{- range seq (sub $lastHeaderLevel $firstHeaderLevel) -}}
                    {{- if in ($.Scratch.Get "bareul") (add . $firstHeaderLevel) }}
                </ul>
                {{- else }}
                </ul>
                </li>
                {{- end -}}
                {{- end }}
                </ul>
            </div>
        </details>
    </div>
</aside>
<script>
    let activeElement;
    let elements;
    window.addEventListener('DOMContentLoaded', function (event) {
        checkTocPosition();

        elements = document.querySelectorAll('h1[id],h2[id],h3[id],h4[id],h5[id],h6[id]');
         // Make the first header active
         activeElement = elements[0];
         const id = encodeURI(activeElement.getAttribute('id')).toLowerCase();
         document.querySelector(`.inner ul li a[href="#${id}"]`).classList.add('active');
     }, false);

    window.addEventListener('resize', function(event) {
        checkTocPosition();
    }, false);

    window.addEventListener('scroll', () => {
        // Check if there is an object in the top half of the screen or keep the last item active
        activeElement = Array.from(elements).find((element) => {
            if ((getOffsetTop(element) - window.pageYOffset) > 0 && 
                (getOffsetTop(element) - window.pageYOffset) < window.innerHeight/2) {
                return element;
            }
        }) || activeElement

        elements.forEach(element => {
             const id = encodeURI(element.getAttribute('id')).toLowerCase();
             if (element === activeElement){
                 document.querySelector(`.inner ul li a[href="#${id}"]`).classList.add('active');
             } else {
                 document.querySelector(`.inner ul li a[href="#${id}"]`).classList.remove('active');
             }
         })
     }, false);

    const main = parseInt(getComputedStyle(document.body).getPropertyValue('--article-width'), 10);
    const toc = parseInt(getComputedStyle(document.body).getPropertyValue('--toc-width'), 10);
    const gap = parseInt(getComputedStyle(document.body).getPropertyValue('--gap'), 10);

    function checkTocPosition() {
        const width = document.body.scrollWidth;

        if (width - main - (toc * 2) - (gap * 4) > 0) {
            document.getElementById("toc-container").classList.add("wide");
        } else {
            document.getElementById("toc-container").classList.remove("wide");
        }
    }

    function getOffsetTop(element) {
        if (!element.getClientRects().length) {
            return 0;
        }
        let rect = element.getBoundingClientRect();
        let win = element.ownerDocument.defaultView;
        return rect.top + win.pageYOffset;   
    }
</script>
{{- end }}

Press CTRL+X, type y to save, and press Enter to exit.

2. Modify the css file

Use the following commands to generate and edit blank.css:

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mkdir assets/css/extended -p \
  && nano assets/css/extended/blank.css

Paste the following content:

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:root {
    --nav-width: 1380px;
    --article-width: 650px;
    --toc-width: 300px;
}

.toc {
    margin: 0 2px 40px 2px;
    border: 1px solid var(--border);
    background: var(--entry);
    border-radius: var(--radius);
    padding: 0.4em;
}

.toc-container.wide {
    position: absolute;
    height: 100%;
    border-right: 1px solid var(--border);
    left: calc((var(--toc-width) + var(--gap)) * -1);
    top: calc(var(--gap) * 2);
    width: var(--toc-width);
}

.wide .toc {
    position: sticky;
    top: var(--gap);
    border: unset;
    background: unset;
    border-radius: unset;
    width: 100%;
    margin: 0 2px 40px 2px;
}

.toc details summary {
    cursor: zoom-in;
    margin-inline-start: 20px;
    padding: 12px 0;
}

.toc details[open] summary {
    font-weight: 500;
}

.toc-container.wide .toc .inner {
    margin: 0;
}

.active {
    font-size: 110%;
    font-weight: 600;
}

.toc ul {
    list-style-type: circle;
}

.toc .inner {
    margin: 0 0 0 20px;
    padding: 0px 15px 15px 20px;
    font-size: 16px;
}

.toc li ul {
    margin-inline-start: calc(var(--gap) * 0.5);
    list-style-type: none;
}

.toc li {
    list-style: none;
    font-size: 0.95rem;
    padding-bottom: 5px;
}

.toc li a:hover {
    color: var(--secondary);
}

Save and exit with the same steps as above — the sidebar TOC modification is now complete.

If you also want the long-press image zoom feature, you can continue adding the following to blank.css:

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img:active {
    transform: scale(1.35,1.35)
}

References

Hugo blog TOC on the side | PaperMod theme

Hugo Sidebar Table of Contents#

Steps#

1. Modify toc.html#

2. Modify the css file#

References#

Hugo Sidebar Table of Contents

Steps

1. Modify toc.html

2. Modify the css file

References