feat(composer): offer to format pasted code as a code block (#107)

When a multi-line paste looks like code, a chip above the composer asks
"That looks like code. Format it as a code block (js)?" with Format as code /
No thanks. It never converts on its own; typing, No, or 8 s dismiss it, and
Settings → Editor → "Offer to Format Pasted Code" turns it off.

- utils/looksLikeCode.ts (pure, tested): ≥ 3 lines and ≥ 2 of indentation
  with depth changes, statement terminators, operators + brackets, keywords,
  monospace/<pre> clipboard HTML, or SQL clause lines. Prose guards: quoted
  replies, URL lists, long sentence-punctuated lines, plain-word lines,
  markdown lists. Language guess only when fairly sure (js/ts/python/sql/
  rust/c/php).
- Accept rebuilds the pasted paragraphs as one code block from their plain
  text (code lines hold text only) and leaves the caret after it.
- Code blocks carry an optional lang → <code class="language-js"> (whitelisted
  identifier only).

Verified in Chromium: pasting a JS function shows the chip; Format as code →
sent formatted_body is <pre><code class="language-js"> with indentation intact;
a three-line prose paste shows no chip; typing after a paste dismisses it.
Unit tests: 14 detector fixtures + 2 output tests; chromium e2e 19 passed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/editor/output.test.ts

@@ -89,3 +89,22 @@ test('markdown: no math conversion inside a backtick span, math outside still co
   assert.ok(/<code[^>]*>\$x\$<\/code>/.test(out), 'backtick span stays literal');
   assert.ok(out.includes('data-mx-maths="y"'));
 });
+
+test('[#107] a code block language becomes class="language-…"', () => {
+  const block = {
+    ...(el(BlockType.CodeBlock, [el(BlockType.CodeLine, [txt('let a = 1;')])]) as object),
+    lang: 'js',
+  } as unknown as Descendant;
+  assert.equal(
+    toMatrixCustomHTML(block, OPTS),
+    '<pre><code class="language-js">let a = 1;\n</code></pre>',
+  );
+});
+
+test('[#107] an unsafe language value is dropped', () => {
+  const block = {
+    ...(el(BlockType.CodeBlock, [el(BlockType.CodeLine, [txt('x')])]) as object),
+    lang: 'js" onclick="x',
+  } as unknown as Descendant;
+  assert.equal(toMatrixCustomHTML(block, OPTS), '<pre><code>x\n</code></pre>');
+});

src/app/components/editor/output.ts

@@ -86,7 +86,10 @@ const elementToCustomHtml = (node: CustomElement, children: string): string => {
     case BlockType.CodeLine:
       return `${children}\n`;
     case BlockType.CodeBlock:
-      return `<pre><code>${children}</code></pre>`;
+      // [Gitea #107] Only a plain identifier is ever put in the class.
+      return node.lang && /^[a-z0-9+#-]{1,20}$/.test(node.lang)
+        ? `<pre><code class="language-${node.lang}">${children}</code></pre>`
+        : `<pre><code>${children}</code></pre>`;
     case BlockType.QuoteLine:
       return `${children}<br/>`;
     case BlockType.BlockQuote:

src/app/components/editor/slate.d.ts

@@ -64,6 +64,8 @@ export type CodeLineElement = {
 };
 export type CodeBlockElement = {
   type: BlockType.CodeBlock;
+  /** [Gitea #107] Optional language → `<code class="language-…">`. */
+  lang?: string;
   children: CodeLineElement[];
 };
 export type QuoteLineElement = {

src/app/features/room/RoomInput.tsx

@@ -13,8 +13,9 @@ import { useAtom, useAtomValue, useSetAtom } from 'jotai';
 import { isKeyHotkey } from 'is-hotkey';
 import { EventType, IContent, MsgType, RelationType, Room } from 'matrix-js-sdk';
 import { ReactEditor } from 'slate-react';
-import { Transforms, Editor } from 'slate';
+import { Transforms, Editor, Element as SlateElement, Node as SlateNode, Range } from 'slate';
 import {
+  Button,
   Box,
   Dialog,
   Icon,
@@ -58,6 +59,7 @@ import {
   getBeginCommand,
   trimCommand,
   getMentions,
+  BlockType,
 } from '../../components/editor';
 import { EmojiBoardTab } from '../../components/emoji-board/types';
 import { UseStateProvider } from '../../components/UseStateProvider';
@@ -99,6 +101,7 @@ import { filesToUploadItems } from '../../utils/uploadItems';
 import { ReplyMediaThumb, hasReplyMedia } from '../../components/message/ReplyMediaThumb';
 import { fulfilledPromiseSettledResult } from '../../utils/common';
 import { useSetting } from '../../state/hooks/settings';
+import { looksLikeCode } from '../../utils/looksLikeCode';
 import { useAlive } from '../../hooks/useAlive';
 import {
   ComposerToolbarButtonKey,
@@ -297,6 +300,48 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
     const [gifPickerEnabled] = useSetting(settingsAtom, 'gifPickerEnabled');
     // [Gitea #103] Privacy: drop tracking params from links on paste and on send.
     const [stripTracking] = useSetting(settingsAtom, 'stripTrackingParams');
+    // [Gitea #107] "Paste as code block?" offer for the top-level blocks a
+    // paste just filled. Dismissed by typing, by No, or after 8 s.
+    const [offerCodePaste] = useSetting(settingsAtom, 'offerCodePaste');
+    const [codeOffer, setCodeOffer] = useState<{ start: number; end: number; lang?: string }>();
+    useEffect(() => {
+      if (!codeOffer) return undefined;
+      const t = window.setTimeout(() => setCodeOffer(undefined), 8000);
+      return () => window.clearTimeout(t);
+    }, [codeOffer]);
+    const acceptCodeOffer = useCallback(() => {
+      if (!codeOffer) return;
+      const { start, end, lang } = codeOffer;
+      setCodeOffer(undefined);
+      if (end >= editor.children.length || start > end) return;
+      const blocks = editor.children.slice(start, end + 1);
+      // Only plain paragraphs (what a paste produces); anything else, leave it.
+      if (blocks.some((b) => !SlateElement.isElement(b) || b.type !== BlockType.Paragraph)) return;
+      // Rebuild from plain text: code lines hold text only, and a paste may
+      // have produced links/mentions.
+      const lines = blocks.map((b) => SlateNode.string(b));
+      Editor.withoutNormalizing(editor, () => {
+        for (let i = end; i >= start; i -= 1) Transforms.removeNodes(editor, { at: [i] });
+        Transforms.insertNodes(
+          editor,
+          {
+            type: BlockType.CodeBlock,
+            lang,
+            children: lines.map((text) => ({ type: BlockType.CodeLine, children: [{ text }] })),
+          },
+          { at: [start] },
+        );
+        if (start + 1 >= editor.children.length) {
+          Transforms.insertNodes(
+            editor,
+            { type: BlockType.Paragraph, children: [{ text: '' }] },
+            { at: [start + 1] },
+          );
+        }
+      });
+      Transforms.select(editor, Editor.start(editor, [start + 1]));
+      ReactEditor.focus(editor);
+    }, [codeOffer, editor]);
     const [stripImageMetadata] = useSetting(settingsAtom, 'stripImageMetadata');
     const showGif = (composerToolbarButtons?.showGif ?? true) && gifPickerEnabled;
     const showLocation = composerToolbarButtons?.showLocation ?? true;
@@ -411,7 +456,26 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
     const handlePaste = useCallback<React.ClipboardEventHandler>(
       (evt) => {
         handleFilePaste(evt);
-        if (evt.defaultPrevented || !stripTracking) return;
+        if (evt.defaultPrevented) return;
+        if (offerCodePaste && editor.selection) {
+          const pasted = evt.clipboardData?.getData('text/plain') ?? '';
+          const startBlock = Range.start(editor.selection).path[0];
+          const startNode = editor.children[startBlock];
+          const inCode =
+            SlateElement.isElement(startNode) && startNode.type === BlockType.CodeBlock;
+          const guess =
+            !inCode && pasted
+              ? looksLikeCode(pasted, evt.clipboardData?.getData('text/html'))
+              : undefined;
+          if (guess?.isCode) {
+            // Slate inserts after this handler returns; read where it ended.
+            window.setTimeout(() => {
+              const endBlock = editor.selection ? Range.end(editor.selection).path[0] : startBlock;
+              setCodeOffer({ start: startBlock, end: endBlock, lang: guess.lang });
+            }, 0);
+          }
+        }
+        if (!stripTracking) return;
         const text = evt.clipboardData?.getData('text/plain');
         if (!text) return;
         const cleaned = stripTrackingParamsInText(text);
@@ -424,7 +488,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
         dt.setData('text/plain', cleaned);
         ReactEditor.insertData(editor, dt);
       },
-      [handleFilePaste, stripTracking, editor],
+      [handleFilePaste, stripTracking, editor, offerCodePaste],
     );
     const dropZoneVisible = useFileDropZone(fileDropContainerRef, handleFiles);
     const { gifApiKey } = useClientConfig();
@@ -853,6 +917,15 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
 
     const handleKeyDown: KeyboardEventHandler = useCallback(
       (evt) => {
+        // Typing (not Tab/arrows/modifiers) dismisses the paste-as-code offer.
+        if (
+          evt.key.length === 1 ||
+          evt.key === 'Backspace' ||
+          evt.key === 'Delete' ||
+          evt.key === 'Enter'
+        ) {
+          setCodeOffer(undefined);
+        }
         if (
           (isKeyHotkey('mod+enter', evt) || (!enterForNewline && isKeyHotkey('enter', evt))) &&
           !isComposing(evt)
@@ -1149,6 +1222,43 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
             </Text>
           </Box>
         )}
+        {codeOffer && (
+          <Box
+            role="group"
+            aria-label="Paste as code block"
+            alignItems="Center"
+            gap="200"
+            wrap="Wrap"
+            style={{
+              margin: `0 ${config.space.S300} ${config.space.S100}`,
+              padding: `${config.space.S100} ${config.space.S200}`,
+              borderRadius: config.radii.R300,
+              background: color.SurfaceVariant.Container,
+              border: `${config.borderWidth.B300} solid ${color.SurfaceVariant.ContainerLine}`,
+            }}
+          >
+            <Icon size="100" src={Icons.BlockCode} style={{ flexShrink: 0 }} />
+            <Text size="T200" style={{ flexGrow: 1 }}>
+              That looks like code. Format it as a code block
+              {codeOffer.lang ? ` (${codeOffer.lang})` : ''}?
+            </Text>
+            <Button size="300" variant="Primary" radii="300" onClick={acceptCodeOffer}>
+              <Text size="B300">Format as code</Text>
+            </Button>
+            <Button
+              size="300"
+              variant="Secondary"
+              fill="None"
+              radii="300"
+              onClick={() => {
+                setCodeOffer(undefined);
+                ReactEditor.focus(editor);
+              }}
+            >
+              <Text size="B300">No thanks</Text>
+            </Button>
+          </Box>
+        )}
         <ScheduledMessagesTray roomId={roomId} />
         <CustomEditor
           editableName={editableName}

src/app/features/settings/general/General.tsx

@@ -1405,6 +1405,7 @@ function ComposerToolbarReorder({
 function Editor() {
   const [enterForNewline, setEnterForNewline] = useSetting(settingsAtom, 'enterForNewline');
   const [isMarkdown, setIsMarkdown] = useSetting(settingsAtom, 'isMarkdown');
+  const [offerCodePaste, setOfferCodePaste] = useSetting(settingsAtom, 'offerCodePaste');
   const [editorToolbar, setEditorToolbar] = useSetting(settingsAtom, 'editorToolbar');
   const [composerToolbarButtons, setComposerToolbarButtons] = useSetting(
     settingsAtom,
@@ -1455,6 +1456,13 @@ function Editor() {
           after={<Switch variant="Primary" value={isMarkdown} onChange={setIsMarkdown} />}
         />
       </SequenceCard>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Offer to Format Pasted Code"
+          description="When something you paste looks like code, offer to turn it into a code block. Nothing changes unless you accept."
+          after={<Switch variant="Primary" value={offerCodePaste} onChange={setOfferCodePaste} />}
+        />
+      </SequenceCard>
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Formatting Toolbar"

src/app/state/settings.ts

@@ -268,6 +268,9 @@ export interface Settings {
   // [Gitea #103] Remove utm_/fbclid/… tracking params from links you paste or
   // send, and from links rendered in the timeline. Local only.
   stripTrackingParams: boolean;
+  // [Gitea #107] After a paste that looks like code, offer to make it a code
+  // block (a chip above the composer; never converts on its own).
+  offerCodePaste: boolean;
   // [Gitea #109] Drop EXIF/XMP/IPTC (GPS, camera, timestamp) from JPEG/PNG/WebP
   // uploads without re-encoding. Default on.
   stripImageMetadata: boolean;
@@ -397,6 +400,7 @@ const defaultSettings: Settings = {
   warnOnUnverifiedDevices: false,
 
   stripTrackingParams: true,
+  offerCodePaste: true,
   stripImageMetadata: true,
   callRejoinAfterRestart: 'ask',
   hapticFeedback: true,

src/app/utils/looksLikeCode.test.ts

@@ -0,0 +1,130 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { looksLikeCode } from './looksLikeCode';
+
+const code = (text: string, html?: string) => looksLikeCode(text, html);
+
+describe('looksLikeCode: code', () => {
+  it('JavaScript', () => {
+    const r = code(`function add(a, b) {
+  const sum = a + b;
+  if (sum > 10) {
+    return sum;
+  }
+  return 0;
+}`);
+    assert.equal(r.isCode, true);
+    assert.equal(r.lang, 'js');
+  });
+
+  it('TypeScript', () => {
+    const r = code(`interface User {
+  id: string;
+  name: string;
+}
+const u: User = { id: '1', name: 'a' };`);
+    assert.deepEqual(r, { isCode: true, lang: 'ts' });
+  });
+
+  it('Python', () => {
+    const r = code(`def greet(name):
+    if not name:
+        return "hi"
+    return f"hi {name}"
+print(greet("bob"))`);
+    assert.deepEqual(r, { isCode: true, lang: 'python' });
+  });
+
+  it('SQL', () => {
+    const r = code(`SELECT id, name
+FROM users
+WHERE active = 1
+ORDER BY name;`);
+    assert.deepEqual(r, { isCode: true, lang: 'sql' });
+  });
+
+  it('Rust', () => {
+    const r = code(`pub fn main() {
+    let mut total = 0;
+    for i in 0..10 {
+        total += i;
+    }
+}`);
+    assert.deepEqual(r, { isCode: true, lang: 'rust' });
+  });
+
+  it('config-ish JSON without a language guess', () => {
+    const r = code(`{
+  "name": "lotus",
+  "version": "1.0.0",
+  "private": true
+}`);
+    assert.equal(r.isCode, true);
+  });
+
+  it('an editor paste carrying monospace HTML counts as a signal', () => {
+    const text = `server {
+  listen 443 ssl;
+  server_name chat.example.org;
+}`;
+    assert.equal(code(text, '<div style="font-family: Menlo, monospace">x</div>').isCode, true);
+  });
+});
+
+describe('looksLikeCode: not code', () => {
+  it('fewer than three lines', () => {
+    assert.equal(code('const a = 1;\nconst b = 2;').isCode, false);
+  });
+
+  it('prose with line breaks', () => {
+    assert.equal(
+      code(`Hey, are we still on for tonight?
+I can bring snacks if someone else handles drinks.
+Let me know what time works, I'm free after six.`).isCode,
+      false,
+    );
+  });
+
+  it('a long paragraph pasted as lines', () => {
+    const line =
+      'This is a long sentence about the plan. It keeps going for a while. And then some more words follow here.';
+    assert.equal(code([line, line, line].join('\n')).isCode, false);
+  });
+
+  it('a markdown list', () => {
+    assert.equal(
+      code(`- milk (2 litres)
+- eggs: a dozen
+- bread, the good kind
+- coffee!`).isCode,
+      false,
+    );
+  });
+
+  it('a quoted reply', () => {
+    assert.equal(
+      code(`> const a = 1;
+> if (a) { run(); }
+> return a;`).isCode,
+      false,
+    );
+  });
+
+  it('a list of links', () => {
+    assert.equal(
+      code(`https://example.org/a?b=1
+https://example.org/c
+https://example.org/d;e`).isCode,
+      false,
+    );
+  });
+
+  it('a log excerpt', () => {
+    assert.equal(
+      code(`2026-09-25 10:00:01 INFO server started
+2026-09-25 10:00:02 INFO listening on 8008
+2026-09-25 10:00:05 WARN slow request 1200ms`).isCode,
+      false,
+    );
+  });
+});

src/app/utils/looksLikeCode.ts

@@ -0,0 +1,111 @@
+/**
+ * [Gitea #107] Decide whether a multi-line paste looks like source code, so the
+ * composer can OFFER to turn it into a code block (never converts on its own).
+ * Pure and unit-tested; tuned to keep prose, lists, quotes and URL lists out.
+ */
+
+export type CodeGuess = {
+  /** Offer "Paste as code block?" */
+  isCode: boolean;
+  /** Language for `class="language-…"`, only when fairly sure. */
+  lang?: string;
+};
+
+const KEYWORDS = [
+  /\bfunction\b/,
+  /\bconst\b/,
+  /\blet\b/,
+  /\bvar\b/,
+  /\breturn\b/,
+  /\bimport\b/,
+  /\bexport\b/,
+  /\bclass\b/,
+  /\bdef\b/,
+  /\bfn\b/,
+  /\bpub\b/,
+  /\bstruct\b/,
+  /\bif \(/,
+  /\bfor \(/,
+  /\bwhile \(/,
+  /#include\b/,
+  /\bSELECT\b/i,
+  /\bFROM\b/i,
+  /<\?php/,
+  /<\//,
+];
+
+const MONO_HTML = /font-family:[^;"]*(mono|courier|consolas|menlo)|<pre[\s>]|<code[\s>]/i;
+
+const indentOf = (line: string): number => {
+  const m = /^[ \t]*/.exec(line);
+  return m ? m[0].replace(/\t/g, '    ').length : 0;
+};
+
+export function guessLanguage(text: string): string | undefined {
+  if (
+    /^\s*(SELECT|INSERT|UPDATE|DELETE|CREATE TABLE)\b/im.test(text) &&
+    /\bFROM\b|\bINTO\b|\bSET\b|\(/i.test(text)
+  )
+    return 'sql';
+  if (/\bpub fn\b|\bfn \w+\(.*\)\s*(->|\{)|\blet mut\b/.test(text)) return 'rust';
+  if (/^\s*#include\s*[<"]/m.test(text)) return 'c';
+  if (
+    /^\s*def \w+\(.*\):\s*$/m.test(text) ||
+    (/^\s*(from \w+ )?import \w+\s*$/m.test(text) && /:\s*$/m.test(text))
+  )
+    return 'python';
+  if (/:\s*(string|number|boolean)\b|\binterface \w+|\btype \w+ =/.test(text) && /[{;]/.test(text))
+    return 'ts';
+  if (/\b(const|let|var)\b|=>/.test(text) && /[{;]/.test(text)) return 'js';
+  if (/^\s*<\?php/m.test(text)) return 'php';
+  return undefined;
+}
+
+export function looksLikeCode(text: string, html?: string): CodeGuess {
+  const lines = text.replace(/\r\n?/g, '\n').split('\n');
+  const nonEmpty = lines.filter((l) => l.trim() !== '');
+  if (nonEmpty.length < 3) return { isCode: false };
+  const n = nonEmpty.length;
+  const share = (pred: (l: string) => boolean) => nonEmpty.filter(pred).length / n;
+
+  // ── Prose guards ──
+  if (share((l) => /^\s*>/.test(l)) > 0.5) return { isCode: false }; // quoted reply
+  if (nonEmpty.every((l) => /^\s*(<)?https?:\/\/\S+(>)?\s*$/.test(l))) return { isCode: false };
+  const avgLen = nonEmpty.reduce((sum, l) => sum + l.length, 0) / n;
+  if (avgLen > 90 && share((l) => (l.match(/[.!?] /g) ?? []).length >= 2) > 0.5) {
+    return { isCode: false };
+  }
+  if (share((l) => /^[\p{L}\p{N}\s'’",.!?-]*$/u.test(l)) > 0.6) return { isCode: false };
+  if (share((l) => /^\s*([-*+]|\d+[.)])\s+\S/.test(l)) > 0.6) return { isCode: false }; // list
+
+  // ── Signals: need two ──
+  let signals = 0;
+  const indented = share((l) => /^( {2,}|\t)/.test(l));
+  let depthChanges = 0;
+  for (let i = 1; i < nonEmpty.length; i += 1) {
+    if (indentOf(nonEmpty[i]) !== indentOf(nonEmpty[i - 1])) depthChanges += 1;
+  }
+  if (indented >= 0.4 && depthChanges >= 2) signals += 1;
+  if (share((l) => /[;{}):,]\s*$/.test(l)) >= 0.3) signals += 1;
+  if (
+    nonEmpty.filter((l) => /=>|->|::|\(\)|[^=!<>]=[^=]/.test(l)).length >= 2 &&
+    /[{}[\]]/.test(text)
+  ) {
+    signals += 1;
+  }
+  if (KEYWORDS.filter((k) => k.test(text)).length >= 2) signals += 1;
+  if (html && MONO_HTML.test(html)) signals += 1;
+  // SQL has few symbols: lines that open with a clause keyword are its shape.
+  if (
+    share((l) =>
+      /^\s*(SELECT|FROM|WHERE|(LEFT |RIGHT |INNER )?JOIN|ORDER BY|GROUP BY|HAVING|INSERT|VALUES|UPDATE|SET|DELETE|LIMIT|AND|OR)\b/.test(
+        l,
+      ),
+    ) >= 0.5
+  ) {
+    signals += 1;
+  }
+
+  if (signals < 2) return { isCode: false };
+  return { isCode: true, lang: guessLanguage(text) };
+}