feat(markdown): render and navigate GFM footnotes

Footnote definitions previously vanished on display. Two underlying issues:

- rehype-sanitize re-clobbered `id` attributes with a second `user-content-`
  prefix while leaving hrefs untouched, so footnote refs/backrefs pointed at
  ids that didn't exist. Disable clobbering (ids are already namespaced by
  remark-rehype) so anchors match their targets.
- Footnote anchors went through the external Link (target="_blank"), opening a
  blank tab instead of navigating. Route in-page `#` anchors through a new
  AnchorLink: scroll within the memo when shown in full, else navigate to the
  memo detail page with the hash, where MemoDetail scrolls it into view.

Also style the footnotes section GitHub-style: thin separator, smaller muted
text, and un-underlined ref/backref links.

web/src/components/MemoContent/MemoMarkdownRenderer.tsx

@@ -19,7 +19,7 @@ import { CodeBlock } from "./CodeBlock";
 import { SANITIZE_SCHEMA } from "./constants";
 import { MarkdownRenderContext, rootMarkdownRenderContext } from "./MarkdownRenderContext";
 import { Mention } from "./Mention";
-import { Blockquote, Heading, HorizontalRule, Image, InlineCode, Link, List, ListItem, Paragraph } from "./markdown";
+import { AnchorLink, Blockquote, Heading, HorizontalRule, Image, InlineCode, Link, List, ListItem, Paragraph } from "./markdown";
 import { Table, TableBody, TableCell, TableHead, TableHeaderCell, TableRow } from "./Table";
 import { Tag } from "./Tag";
 import { TaskListItem } from "./TaskListItem";
@@ -28,6 +28,10 @@ import { TrustedIframe } from "./TrustedIframe";
 interface MemoMarkdownRendererProps {
   content: string;
   resolvedMentionUsernames: Set<string>;
+  /** Resource name of the memo (e.g. `memos/abc123`), used to target footnote links at the detail page. */
+  memoName?: string;
+  /** Whether the memo is rendered as a collapsed feed card. */
+  compact?: boolean;
 }
 
 function getMentionUsername(node: Element, children?: React.ReactNode): string {
@@ -49,7 +53,7 @@ function getMentionUsername(node: Element, children?: React.ReactNode): string {
   return "";
 }
 
-export const MemoMarkdownRenderer = ({ content, resolvedMentionUsernames }: MemoMarkdownRendererProps) => {
+export const MemoMarkdownRenderer = ({ content, resolvedMentionUsernames, memoName, compact }: MemoMarkdownRendererProps) => {
   const markdownComponents: Components = {
     input: ({ node, ...inputProps }) => {
       if (node && isTaskListItemElement(node)) {
@@ -107,7 +111,22 @@ export const MemoMarkdownRenderer = ({ content, resolvedMentionUsernames }: Memo
       </List>
     ),
     li: ({ children, ...props }) => <ListItem {...props}>{children}</ListItem>,
-    a: ({ children, ...props }) => <Link {...props}>{children}</Link>,
+    a: ({ children, href, ...props }) => {
+      // In-page anchors (footnote refs/backrefs, heading links) navigate within the memo rather
+      // than opening a new tab; everything else is treated as an external link.
+      if (typeof href === "string" && href.startsWith("#")) {
+        return (
+          <AnchorLink href={href} memoName={memoName} compact={compact} {...props}>
+            {children}
+          </AnchorLink>
+        );
+      }
+      return (
+        <Link href={href} {...props}>
+          {children}
+        </Link>
+      );
+    },
     code: ({ children, ...props }) => <InlineCode {...props}>{children}</InlineCode>,
     iframe: TrustedIframe,
     img: (props) => <Image {...props} />,

web/src/components/MemoContent/constants.ts

@@ -58,6 +58,11 @@ export const isTrustedIframeSrc = (src: string): boolean => TRUSTED_IFRAME_SRC_P
  */
 export const SANITIZE_SCHEMA = {
   ...defaultSchema,
+  // Don't re-prefix `id`/`name` attributes. remark-rehype already namespaces footnote ids with
+  // `user-content-` and emits matching `#user-content-…` hrefs; the default schema's clobbering
+  // would prepend a *second* `user-content-` to the ids only (not the hrefs), breaking in-page
+  // navigation. Leaving ids untouched keeps footnote (and heading) anchors pointing at real targets.
+  clobber: [],
   attributes: {
     ...defaultSchema.attributes,
     img: [...(defaultSchema.attributes?.img || []), "height", "width"],

web/src/components/MemoContent/index.tsx

@@ -33,6 +33,12 @@ const MemoContent = (props: MemoContentProps) => {
           "[&_.katex-display]:max-w-full",
           "[&_.katex-display]:overflow-x-auto",
           "[&_.katex-display]:overflow-y-hidden",
+          // Footnotes: quiet GitHub-style footer — thin separator, smaller muted text, unobtrusive links.
+          "[&_.footnotes]:mt-4 [&_.footnotes]:border-t [&_.footnotes]:border-border [&_.footnotes]:pt-2",
+          "[&_.footnotes]:text-sm [&_.footnotes]:text-muted-foreground",
+          // GitHub renders footnote ref/backref links without an underline (underline on hover only).
+          "[&_[data-footnote-ref]]:no-underline [&_[data-footnote-ref]:hover]:underline",
+          "[&_.data-footnote-backref]:no-underline [&_.data-footnote-backref:hover]:underline",
           showCompactMode === "ALL" && "overflow-hidden",
           contentClassName,
         )}
@@ -40,7 +46,12 @@ const MemoContent = (props: MemoContentProps) => {
         onMouseUp={onClick}
         onDoubleClick={onDoubleClick}
       >
-        <MemoMarkdownRenderer content={content} resolvedMentionUsernames={resolvedMentionUsernames} />
+        <MemoMarkdownRenderer
+          content={content}
+          resolvedMentionUsernames={resolvedMentionUsernames}
+          memoName={props.memoName}
+          compact={Boolean(props.compact)}
+        />
         {showCompactMode === "ALL" && (
           <div
             className={cn(

web/src/components/MemoContent/markdown/AnchorLink.tsx

@@ -0,0 +1,54 @@
+import { Link } from "react-router-dom";
+import { markdownStyles } from "@/lib/markdownStyles";
+import { cn } from "@/lib/utils";
+import type { ReactMarkdownProps } from "./types";
+
+interface AnchorLinkProps extends React.AnchorHTMLAttributes<HTMLAnchorElement>, ReactMarkdownProps {
+  href: string;
+  /** Resource name of the enclosing memo (e.g. `memos/abc123`), when known. */
+  memoName?: string;
+  /** Whether the memo is rendered as a collapsed feed card. */
+  compact?: boolean;
+  children: React.ReactNode;
+}
+
+/**
+ * Renders in-page anchors (footnote references/backrefs, heading links — any `href="#…"`).
+ *
+ * When the target lives in a fully-rendered memo (detail, share, preview, expanded feed card),
+ * we scroll to it within that memo's own container — scoped so duplicate footnote ids across a
+ * feed can't send us to the wrong memo. When the memo is a collapsed feed card the footnote is
+ * below the fold, so we fall back to navigating to the memo detail page (with the hash), where
+ * MemoDetail scrolls the target into view.
+ */
+export const AnchorLink = ({ href, memoName, compact, children, className, node: _node, ...props }: AnchorLinkProps) => {
+  const handleClick = (event: React.MouseEvent<HTMLAnchorElement>) => {
+    if (compact) return; // Let the link navigate to the detail page.
+    const id = decodeURIComponent(href.slice(1));
+    if (!id) return;
+    // Scope the lookup to this memo's own container so duplicate footnote ids elsewhere in a feed
+    // can't steal the scroll.
+    const root = event.currentTarget.closest("[data-memo-content]");
+    const target = root?.querySelector(`#${CSS.escape(id)}`);
+    if (target) {
+      event.preventDefault();
+      target.scrollIntoView({ behavior: "smooth", block: "center" });
+    }
+  };
+
+  const classes = cn(markdownStyles.link, className);
+
+  if (memoName) {
+    return (
+      <Link to={`/${memoName}${href}`} onClick={handleClick} className={classes} {...props}>
+        {children}
+      </Link>
+    );
+  }
+
+  return (
+    <a href={href} onClick={handleClick} className={classes} {...props}>
+      {children}
+    </a>
+  );
+};

web/src/components/MemoContent/markdown/index.ts

@@ -1,3 +1,4 @@
+export { AnchorLink } from "./AnchorLink";
 export { Blockquote } from "./Blockquote";
 export { Heading } from "./Heading";
 export { HorizontalRule } from "./HorizontalRule";

web/src/components/MemoContent/types.ts

@@ -2,6 +2,8 @@ import type React from "react";
 
 export interface MemoContentProps {
   content: string;
+  /** Resource name of the memo (e.g. `memos/abc123`). Enables footnote links to target the memo detail page. */
+  memoName?: string;
   compact?: boolean;
   className?: string;
   contentClassName?: string;

web/src/components/MemoView/components/MemoBody.tsx

@@ -36,6 +36,7 @@ const MemoBody: React.FC<MemoBodyProps> = ({ compact }) => {
       >
         <MemoContent
           key={memo.name}
+          memoName={memo.name}
           content={memo.content}
           onClick={handleMemoContentClick}
           onDoubleClick={handleMemoContentDoubleClick}

web/src/pages/MemoDetail.tsx

@@ -1,6 +1,6 @@
 import { Code, ConnectError } from "@connectrpc/connect";
 import { ArrowUpLeftFromCircleIcon } from "lucide-react";
-import { useEffect, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { Link, Navigate, useLocation, useParams } from "react-router-dom";
 import MemoCommentSection from "@/components/MemoCommentSection";
 import { MentionResolutionProvider } from "@/components/MemoContent/MentionResolutionContext";
@@ -57,11 +57,17 @@ const MemoDetail = () => {
     enabled: !!memo,
   });
 
+  // Scroll to the hash target once it's in the DOM. The effect re-runs as the memo loads (footnote
+  // anchors) and as comments arrive (comment anchors), since the target may render in either; the
+  // ref guards against re-scrolling the same hash on every later comments page-load.
+  const scrolledHashRef = useRef("");
   useEffect(() => {
-    if (!hash || comments.length === 0) return;
+    if (!hash || scrolledHashRef.current === hash) return;
-    const el = document.getElementById(hash.slice(1));
+    const el = document.getElementById(decodeURIComponent(hash.slice(1)));
-    el?.scrollIntoView({ behavior: "smooth", block: "center" });
+    if (!el) return;
-  }, [hash, comments]);
+    scrolledHashRef.current = hash;
+    el.scrollIntoView({ behavior: "smooth", block: "center" });
+  }, [hash, memo, comments]);
 
   if (isShareMode) {
     const isNotFound = error instanceof ConnectError && (error.code === Code.NotFound || error.code === Code.Unauthenticated);