web: fix the outline anchors, level the outline, and fold the sidebar chapters

content/concepts/keplerian-elements.zh.md

@@ -19,7 +19,7 @@ summary: 六个经典轨道元素、二体解与三种近点角。
 | 近地点幅角 | `omega` | 轨道面内从升交点到近地点的角度 | rad |
 | 真近点角 | `nu` | 从近地点到卫星的角度 | rad |
 
-前两个给出形状与大小，接下来两个给出轨道面的朝向，最后两个给出近地点在轨道面内的位置与卫星沿轨道的位<EFBFBD>置。
+前两个给出形状与大小，接下来两个给出轨道面的朝向，最后两个给出近地点在轨道面内的位置与卫星沿轨道的位置。
 
 轨道是以地心为一个焦点的椭圆，即开普勒第一定律。离地球最近的点是近地点，最远的是远地点。
 
@@ -110,7 +110,7 @@ M = E - e sin(E)
 | 赤道元素 | 状态矢量 | 二体 | 是 | 否 |
 | 时间网格上的位置 | 任意传播器 | 任意 | 是 | 插值，见 [[ephemeris-interpolation]] |
 
-赤道元素避免了经典元素在倾角为零和偏心率<EFBFBD>为零时的奇异。此处使用经典形式，因为两行元素格式与已发表文献都采用这种形式，且地球轨道几乎总有非零倾角。
+赤道元素避免了经典元素在倾角为零和偏心率为零时的奇异。此处使用经典形式，因为两行元素格式与已发表文献都采用这种形式，且地球轨道几乎总有非零倾角。
 
 ## 来源
 

docs/design.md

@@ -325,6 +325,27 @@ settles into a roughly round cluster while the canvas is wide, so the layout
 is widened to the canvas shape before it is fitted; the change is a uniform
 scale, which leaves relative distances readable.
 
+## Anchors and outlines
+
+A heading anchor is reduced from the heading text, and a table of contents
+carries that anchor. Two rules make the link land where the reader expects.
+
+The reduction **keeps the letters of every script**. Keeping only the ASCII
+ones meant every Chinese heading reduced to the same fallback, so the whole
+table of contents pointed at the first section and clicking an entry always
+scrolled to the top of the document. The document text and the outline are
+reduced by the same rule, so an entry names an anchor the document carries.
+
+A repeated heading is **numbered on both sides**. Two headings with the same
+text would otherwise share one anchor, and the second entry would scroll to
+the first. The document and the outline number the repeat the same way, so
+the mapping stays exact.
+
+An outline carries the **level** of each heading, so a nested heading is
+indented under its parent rather than listed beside it. A fold control is
+shown only for a heading that has a child, because a control that hides
+nothing is worse than no control.
+
 ## Language coverage
 
 A user facing string falls into three classes, and each is handled

frontend/src/App.svelte

@@ -45,6 +45,16 @@
   let searchLoading = $state(false);
   let status = $state<string | null>(null);
 
+  /** The chapters whose documents are hidden in the sidebar. */
+  let collapsed = $state<string[]>([]);
+
+  /** Folds a chapter, or unfolds it when it is already folded. */
+  function toggleChapter(id: string) {
+    collapsed = collapsed.includes(id)
+      ? collapsed.filter((entry) => entry !== id)
+      : [...collapsed, id];
+  }
+
   /** Loads the content every page shares. */
   async function loadShared() {
     try {
@@ -178,15 +188,27 @@
       {#if index}
         <div class="sidebar-section">
           <h2>{tr('nav.documents', lang)}</h2>
-          {#each index.chapters as chapter}
+          {#each index.chapters as chapter (chapter.id)}
-            <div class="sidebar-chapter">
+            <div class="sidebar-chapter" class:collapsed={collapsed.includes(chapter.id)}>
-              <div class="sidebar-chapter-title">{pick(chapter.title, lang)}</div>
+              <div class="sidebar-chapter-head">
-              {#each chapter.documents as entry}
+                <a class="sidebar-chapter-title" href="#/docs" title="{pick(chapter.title, lang)}">
-                <a
+                  {pick(chapter.title, lang)}
-                  class="item"
+                </a>
-                  class:active={route.name === 'docs' && route.id === entry.id}
+                <button
-                  href={`#/docs/${entry.id}`}>{pick(entry.title, lang)}</a>
+                  class="sidebar-chapter-toggle"
-              {/each}
+                  aria-expanded={!collapsed.includes(chapter.id)}
+                  aria-label="{tr('nav.toggleChapter', lang)}: {pick(chapter.title, lang)}"
+                  onclick={() => toggleChapter(chapter.id)}
+                >▾</button>
+              </div>
+              {#if !collapsed.includes(chapter.id)}
+                {#each chapter.documents as entry}
+                  <a
+                    class="item"
+                    class:active={route.name === 'docs' && route.id === entry.id}
+                    href={`#/docs/${entry.id}`}>{pick(entry.title, lang)}</a>
+                {/each}
+              {/if}
             </div>
           {/each}
         </div>

frontend/src/app.css

@@ -227,14 +227,60 @@ hr {
 }
 
 .sidebar-chapter {
-  margin-bottom: 0.75rem;
+  margin-bottom: 0.5rem;
+}
+
+/* The chapter head is a row: the title is a link to the chapter listing
+   and the chevron folds the group. They sit apart so the two do not look
+   like one control. */
+.sidebar-chapter-head {
+  display: flex;
+  align-items: center;
+  gap: 0.1rem;
+  margin-bottom: 0.15rem;
+  padding-left: 0.35rem;
+  border-left: 2px solid var(--border-strong);
 }
 
 .sidebar-chapter-title {
-  font-size: 0.82rem;
+  flex: 1 1 auto;
-  color: var(--text-muted);
+  font-size: 0.78rem;
-  margin: 0 0 0.2rem 0.5rem;
+  color: var(--text);
-  font-weight: 600;
+  margin: 0;
+  font-weight: 700;
+  letter-spacing: 0.02em;
+  text-transform: uppercase;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.sidebar-chapter-title:hover {
+  color: var(--accent);
+  text-decoration: none;
+}
+
+.sidebar-chapter-toggle {
+  flex: 0 0 auto;
+  width: 1.1rem;
+  height: 1.1rem;
+  padding: 0;
+  border: none;
+  background: none;
+  color: var(--text-faint);
+  font-size: 0.65rem;
+  line-height: 1;
+  cursor: pointer;
+  transition: transform 0.12s ease;
+}
+
+.sidebar-chapter.collapsed .sidebar-chapter-toggle {
+  transform: rotate(-90deg);
+}
+
+.sidebar-chapter-toggle:hover {
+  color: var(--accent);
+  background: none;
 }
 
 .sidebar a.item {
@@ -1061,11 +1107,33 @@ tbody tr:hover {
 }
 
 .tool-card h4 {
+  display: flex;
+  align-items: baseline;
+  gap: 0.35rem;
   margin: 0 0 0.35rem;
   font-size: 0.98rem;
   color: var(--accent);
 }
 
+/* The chevron marks the card as an entry point to a page of its own, which
+   is what a reader cannot otherwise tell from a heading. */
+.tool-go {
+  color: var(--text-faint);
+  font-size: 1.05rem;
+  line-height: 1;
+  transition: transform 0.12s ease, color 0.12s ease;
+}
+
+.tool-card:hover .tool-go {
+  color: var(--accent);
+  transform: translateX(2px);
+}
+
+/* The form a tool opens, given room to breathe under the index. */
+.tool-form {
+  scroll-margin-top: 4.5rem;
+}
+
 .tool-card p {
   margin: 0;
   color: var(--text-muted);
@@ -1077,6 +1145,65 @@ tbody tr:hover {
   background: var(--accent-soft);
 }
 
+/* The table of contents beside a document. */
+
+.toc-list {
+  list-style: none;
+  padding: 0;
+  margin: 0;
+}
+
+/* A deeper heading is indented, so the levels of the document are visible
+   without reading the text. */
+.toc-item {
+  display: flex;
+  align-items: baseline;
+  gap: 0.15rem;
+  margin: 0.1rem 0;
+}
+
+.toc-item.level-3 {
+  padding-left: 0.85rem;
+}
+
+.toc-item.level-4 {
+  padding-left: 1.7rem;
+}
+
+.toc-item.level-5,
+.toc-item.level-6 {
+  padding-left: 2.55rem;
+}
+
+.toc-toggle {
+  flex: 0 0 auto;
+  width: 1rem;
+  height: 1rem;
+  padding: 0;
+  border: none;
+  background: none;
+  color: var(--text-faint);
+  font-size: 0.7rem;
+  line-height: 1;
+  cursor: pointer;
+  transition: transform 0.12s ease;
+}
+
+.toc-toggle.collapsed {
+  transform: rotate(-90deg);
+}
+
+.toc-toggle:hover {
+  color: var(--accent);
+  background: none;
+}
+
+.toc-link {
+  color: var(--text-muted);
+  font-size: 0.85rem;
+  line-height: 1.4;
+}
+
 /* A row of links that wrap, used for the item filter results. */
 .chip-row {
   display: flex;

frontend/src/lib/i18n.ts

@@ -108,6 +108,7 @@ const STRINGS: Record<string, Text> = {
   'nav.api': { zh: '接口文档', en: 'Interface' },
   'nav.tools': { zh: '在线工具', en: 'Online tools' },
   'nav.documents': { zh: '文档', en: 'Documents' },
+  'nav.toggleChapter': { zh: '展开或折叠该章', en: 'Expand or collapse the chapter' },
   'search.placeholder': { zh: '搜索文档', en: 'Search the documentation' },
   'search.button': { zh: '搜索', en: 'Search' },
   'search.title': { zh: '搜索', en: 'Search' },
@@ -171,6 +172,7 @@ const STRINGS: Record<string, Text> = {
   'docs.back': { zh: '返回列表', en: 'Back to the listing' },
   'docs.readFirst': { zh: '先读：', en: 'Read first:' },
   'docs.onThisPage': { zh: '本页目录', en: 'On this page' },
+  'docs.toggle': { zh: '展开或折叠该节', en: 'Expand or collapse the section' },
   'docs.noSections': { zh: '无小节。', en: 'No sections.' },
   'docs.translationMissing': {
     zh: '本文档暂无中文版本，显示英文原文。',
@@ -295,6 +297,7 @@ const STRINGS: Record<string, Text> = {
   'kind.type': { zh: '类型别名', en: 'type' },
 
   'api.itemCount': { zh: '个接口项', en: 'items' },
+  'tools.open': { zh: '打开', en: 'Open' },
   'tools.choose': { zh: '共 {count} 个工具，按类别分组。', en: '{count} tools, grouped by category.' },
 };
 

frontend/src/lib/types.ts

@@ -34,6 +34,13 @@ export interface Health {
   frontend_embedded: boolean;
 }
 
+/** One section heading of a document. */
+export interface DocumentHeading {
+  anchor: string;
+  text: string;
+  level: number;
+}
+
 /** One language form of a document. */
 export interface DocumentForm {
   language: 'zh' | 'en';
@@ -41,7 +48,8 @@ export interface DocumentForm {
   summary: string;
   html: string;
   markdown: string;
-  headings: { anchor: string; text: string }[];
+  /** Section headings, each with the anchor, text, and level. */
+  headings: { anchor: string; text: string; level: number }[];
 }
 
 /** One document, with a rendered form per embedded language. */

frontend/src/lib/uniformCards.ts

@@ -12,6 +12,15 @@
  * height is a property of the rendering, not of the content.
  */
 
+/**
+ * The cards the measurement covers.
+ *
+ * The interface page groups its items under a kind and lays them out in its
+ * own grid, so both grids take part; a card in either is made the height of
+ * the tallest.
+ */
+const CARD_SELECTOR = '.grid.cards > .card, .item-grid > .api-item';
+
 /**
  * Returns the height of the tallest card under `root`, in pixels, or `0` when
  * there is no card.
@@ -20,7 +29,7 @@ export function tallestCardHeight(root: ParentNode): number {
   let tallest = 0;
   // The card is laid out at its natural height here, because the uniform
   // height is not applied yet.
-  for (const card of root.querySelectorAll<HTMLElement>('.grid.cards > .card')) {
+  for (const card of root.querySelectorAll<HTMLElement>(CARD_SELECTOR)) {
     tallest = Math.max(tallest, card.getBoundingClientRect().height);
   }
   return Math.ceil(tallest);
@@ -31,7 +40,7 @@ export function tallestCardHeight(root: ParentNode): number {
  * inline height when there is no card.
  */
 export function applyUniformHeight(root: ParentNode): void {
-  const cards = [...root.querySelectorAll<HTMLElement>('.grid.cards > .card')];
+  const cards = [...root.querySelectorAll<HTMLElement>(CARD_SELECTOR)];
   if (cards.length === 0) {
     return;
   }

frontend/src/pages/Document.svelte

@@ -11,7 +11,7 @@
    * the reader's language is shown, with the English form as the fallback
    * when a translation is absent.
    */
-  import type { Document, DocumentForm } from '../lib/types';
+  import type { Document, DocumentForm, DocumentHeading } from '../lib/types';
   import { navigate } from '../lib/router';
   import { language, pickList, setLanguage, tr, type Language } from '../lib/i18n';
 
@@ -38,6 +38,47 @@
     !!document && !!form && form.language !== lang,
   );
 
+  /** Which top level sections of the table of contents are expanded. */
+  let openSections = $state<Record<string, boolean>>({});
+
+  /**
+   * Returns `true` when a heading has a child under it.
+   *
+   * A fold control is only shown for a heading that actually has children,
+   * so a flat document does not carry controls that do nothing.
+   */
+  function hasChildren(heading: DocumentHeading, index: number): boolean {
+    const next = form?.headings[index + 1];
+    return next !== undefined && next.level > heading.level;
+  }
+
+  /**
+   * Returns `true` when a heading is visible in the table of contents.
+   *
+   * A heading is visible when it is a top level entry, when its section is
+   * expanded, or when it is the first child of a section so that a section
+   * never looks empty while it is collapsed.
+   */
+  function isVisible(heading: DocumentHeading, index: number): boolean {
+    if (heading.level <= 2) return true;
+    const parent = form?.headings
+      .slice(0, index)
+      .reverse()
+      .find((candidate) => candidate.level <= 2);
+    if (!parent) return true;
+    const open = openSections[parent.anchor];
+    if (open !== false) return true;
+    // Keep the first child visible so a collapsed section still shows where
+    // it starts.
+    const firstChild = form?.headings.findIndex((candidate) => candidate.level > 2);
+    return firstChild === index;
+  }
+
+  /** Flips a section open or closed. */
+  function toggleSection(anchor: string) {
+    openSections = { ...openSections, [anchor]: openSections[anchor] === false };
+  }
+
   /** True when a form in the other language is available to switch to. */
   const otherLanguage = $derived.by((): Language | null => {
     if (!document || !form) return null;
@@ -143,17 +184,27 @@
     {#if form.headings.length > 0}
       <aside class="doc-toc">
         <h2 style="font-size: 0.72rem;">{tr('docs.onThisPage', lang)}</h2>
-        <ul style="list-style: none; padding-left: 0;">
+        <ul class="toc-list">
-          {#each form.headings as heading}
+          {#each form.headings as heading, index (heading.anchor)}
-            <li style="margin: 0.2rem 0;">
+            {#if isVisible(heading, index)}
-              <a
+              <li class="toc-item level-{heading.level}">
-                href={`#${heading.anchor}`}
+                {#if hasChildren(heading, index)}
-                style="color: var(--text-muted); font-size: 0.85rem;"
+                  <button
-                onclick={(event) => {
+                    class="toc-toggle"
-                  event.preventDefault();
+                    class:collapsed={openSections[heading.anchor] === false}
-                  scrollToSection(heading.anchor);
+                    aria-expanded={openSections[heading.anchor] !== false}
-                }}>{heading.text}</a>
+                    aria-label="{tr('docs.toggle', lang)}: {heading.text}"
-            </li>
+                    onclick={() => toggleSection(heading.anchor)}>▾</button>
+                {/if}
+                <a
+                  class="toc-link"
+                  href="#{heading.anchor}"
+                  onclick={(event) => {
+                    event.preventDefault();
+                    scrollToSection(heading.anchor);
+                  }}>{heading.text}</a>
+              </li>
+            {/if}
           {/each}
         </ul>
       </aside>

frontend/src/pages/Tools.svelte

@@ -74,6 +74,19 @@
     values = defaultsFor(found);
   });
 
+  /**
+   * Moves the form into view after a tool is chosen.
+   *
+   * The index and the form share the page, so a reader who picks a tool from
+   * the list would otherwise stay on the index and not see the form. This
+   * runs only when the route names a tool, which is what a click does.
+   */
+  let form = $state<HTMLElement | null>(null);
+  $effect(() => {
+    if (!id || !form) return;
+    form.scrollIntoView({ behavior: 'smooth', block: 'start' });
+  });
+
   /** Runs the selected tool. */
   async function run() {
     if (!tool) return;
@@ -147,8 +160,12 @@
             class="card tool-card"
             class:selected={id === candidate.id}
             href={`#/tools/${candidate.id}`}
+            aria-label="{tr('tools.open', lang)}: {pick(candidate.name, lang)}"
           >
-            <h4>{pick(candidate.name, lang)}</h4>
+            <h4>
+              {pick(candidate.name, lang)}
+              <span class="tool-go" aria-hidden="true">›</span>
+            </h4>
             <p>{pick(candidate.summary, lang)}</p>
           </a>
         {/each}
@@ -161,7 +178,7 @@
       <p>{tr('tools.select', lang)}</p>
     </div>
   {:else}
-    <section class="card" style="margin-top: 1.5rem;">
+    <section class="card tool-form" bind:this={form} style="margin-top: 1.5rem;">
       <header style="display: flex; align-items: baseline; gap: 0.75rem; flex-wrap: wrap;">
         <h2 style="margin: 0; border: none;">{pick(tool.name, lang)}</h2>
         <span class="badge">{pick(tool.category, lang)}</span>

src/api/mod.rs

@@ -191,7 +191,13 @@ impl Router {
                         "headings" => variant
                             .headings
                             .iter()
-                            .map(|(anchor, text)| json_object! { "anchor" => anchor.clone(), "text" => text.clone() })
+                            .map(|heading| {
+                            json_object! {
+                                "anchor" => heading.anchor.clone(),
+                                "text" => heading.text.clone(),
+                                "level" => heading.level as f64,
+                            }
+                        })
                             .collect::<Vec<Value>>(),
                     });
                 }

src/docs/document.rs

@@ -60,8 +60,8 @@ pub struct DocumentVariant {
     pub markdown: String,
     /// Rendered HTML.
     pub html: String,
-    /// Section headings, in order, as pairs of anchor identifier and text.
+    /// Section headings, in order.
-    pub headings: Vec<(String, String)>,
+    pub headings: Vec<Heading>,
     /// Concept identifiers referenced by the body.
     pub references: Vec<String>,
 }
@@ -279,9 +279,8 @@ fn split_list(value: &str) -> Vec<String> {
         .collect()
 }
 
-/// Collects the headings of a Markdown body as pairs of anchor and text.
+fn collect_headings(markdown: &str) -> Vec<Heading> {
-fn collect_headings(markdown: &str) -> Vec<(String, String)> {
+    let mut headings: Vec<Heading> = Vec::new();
-    let mut headings = Vec::new();
     let mut in_code = false;
     for line in markdown.lines() {
         let trimmed = line.trim_end();
@@ -298,20 +297,53 @@ fn collect_headings(markdown: &str) -> Vec<(String, String)> {
             .count();
         if (1..=6).contains(&hashes) && trimmed[hashes..].starts_with(' ') {
             let text = trimmed[hashes..].trim();
-            headings.push((slug(text), text.to_string()));
+            // The anchor must match the one the renderer emits, and two
+            // headings can reduce to the same text, so a repeat is numbered
+            // the way the renderer numbers it.
+            let base = slug(text);
+            let repeats = headings
+                .iter()
+                .filter(|heading| heading.base == base)
+                .count();
+            let anchor = if repeats == 0 {
+                base.clone()
+            } else {
+                format!("{base}-{}", repeats + 1)
+            };
+            headings.push(Heading {
+                base,
+                anchor,
+                text: text.to_string(),
+                level: hashes as u32,
+            });
         }
     }
     headings
 }
 
+/// One heading of a document.
+#[derive(Debug, Clone)]
+pub struct Heading {
+    /// The anchor before the uniqueness suffix, used to detect a repeat.
+    pub base: String,
+    /// The anchor the document carries for this heading.
+    pub anchor: String,
+    /// The heading text.
+    pub text: String,
+    /// The heading level, from one to six.
+    pub level: u32,
+}
+
 /// Returns a URL friendly slug, matching the anchors the Markdown renderer
 /// emits.
 fn slug(text: &str) -> String {
     let mut slug = String::with_capacity(text.len());
     let mut last_dash = true;
     for character in text.chars() {
-        if character.is_ascii_alphanumeric() {
+        if character.is_alphanumeric() {
-            slug.push(character.to_ascii_lowercase());
+            for lowered in character.to_lowercase() {
+                slug.push(lowered);
+            }
             last_dash = false;
         } else if !last_dash {
             slug.push('-');
@@ -889,7 +921,7 @@ BSTAR 项。
         assert_eq!(document.prerequisites, vec!["tle"]);
         assert_eq!(document.references(), vec!["tle"]);
         assert_eq!(document.english.headings.len(), 2);
-        assert_eq!(document.english.headings[1].0, "drag");
+        assert_eq!(document.english.headings[1].anchor, "drag");
         assert!(document.english.html.contains("<h1"));
     }
 
@@ -904,7 +936,7 @@ BSTAR 项。
                 .english
                 .headings
                 .iter()
-                .any(|(_, text)| text == "SGP4")
+                .any(|heading| heading.text == "SGP4")
         );
 
         // A body that repeats the metadata title loses the repeated heading.
@@ -915,7 +947,7 @@ BSTAR 项。
             !variant
                 .headings
                 .iter()
-                .any(|(_, text)| text == "SGP4 propagation")
+                .any(|heading| heading.text == "SGP4 propagation")
         );
         assert!(variant.html.contains("Body."));
     }

src/docs/markdown.rs

@@ -42,6 +42,10 @@ pub fn render(markdown: &str) -> String {
 /// shortcut link whose name is not in the set is left as a code span, so a
 /// doc comment can write `[`0, 1`]` without producing a link to nothing.
 pub fn render_with(markdown: &str, items: Option<&std::collections::HashSet<String>>) -> String {
+    // Anchors must be unique within the document: two headings that reduce
+    // to the same text would otherwise share one anchor, and the second
+    // entry of a table of contents would scroll to the first.
+    let mut seen: std::collections::HashMap<String, usize> = std::collections::HashMap::new();
     let lines: Vec<&str> = markdown.lines().collect();
     let mut output = String::with_capacity(markdown.len() + markdown.len() / 2);
     let mut index = 0;
@@ -70,7 +74,17 @@ pub fn render_with(markdown: &str, items: Option<&std::collections::HashSet<Stri
 
         if let Some(level) = heading_level(trimmed) {
             let text = trimmed[level..].trim();
-            let id = slug(text);
+            // Two headings that reduce to the same text must not share one
+            // anchor, or every later entry of a table of contents scrolls
+            // to the first of them.
+            let base = slug(text);
+            let count = seen.entry(base.clone()).or_insert(0);
+            *count += 1;
+            let id = if *count == 1 {
+                base
+            } else {
+                format!("{base}-{count}")
+            };
             output.push_str(&format!(
                 "<h{level} id=\"{}\">{}<a class=\"anchor\" href=\"#{id}\" aria-hidden=\"true\">#</a></h{level}>\n",
                 escape(&id),
@@ -609,8 +623,14 @@ fn slug(text: &str) -> String {
     let mut slug = String::with_capacity(text.len());
     let mut last_dash = true;
     for character in text.chars() {
-        if character.is_ascii_alphanumeric() {
+        // Letters of any script are kept, so a Chinese heading keeps its own
-            slug.push(character.to_ascii_lowercase());
+        // characters instead of collapsing to the fallback. Keeping only the
+        // ASCII ones gave every Chinese heading the same anchor, so the
+        // table of contents pointed every entry at the first section.
+        if character.is_alphanumeric() {
+            for lowered in character.to_lowercase() {
+                slug.push(lowered);
+            }
             last_dash = false;
         } else if !last_dash {
             slug.push('-');
@@ -791,6 +811,24 @@ mod tests {
         assert!(!html.contains("data-api"), "{html}");
     }
 
+    #[test]
+    fn a_chinese_heading_keeps_its_own_anchor() {
+        // Only ASCII letters were kept, so every Chinese heading reduced to
+        // the fallback and the whole table of contents pointed at the first
+        // section.
+        let html = render("# 时间尺度\n\n正文。\n\n# 单位约定\n\n更多。\n");
+        assert!(html.contains("id=\"时间尺度\""), "{html}");
+        assert!(html.contains("id=\"单位约定\""), "{html}");
+        assert!(!html.contains("id=\"section\""), "{html}");
+    }
+
+    #[test]
+    fn a_repeated_heading_gets_a_distinct_anchor() {
+        let html = render("# 说明\n\n甲。\n\n# 说明\n\n乙。\n");
+        assert!(html.contains("id=\"说明\""), "{html}");
+        assert!(html.contains("id=\"说明-2\""), "{html}");
+    }
+
     #[test]
     fn renders_concept_links() {
         let html = render("See [[sgp4]] and [[tle|the element set]].\n");