fix: inline code input rule deletes character before opening backtick (#1923)

The upstream TipTap Code extension input rule regex /(^|[^`])`([^`]+)`(?!`)$/
uses a capture group (^|[^`]) that includes the character preceding the
opening backtick in the full match. When markInputRule processes this,
it deletes everything from the match start to the code content, which
removes that preceding character along with the backtick delimiters.

For example, typing foo(`bar` would result in foo`bar` (formatted)
instead of the expected foo(`bar` (formatted) — the ( is lost.

Fix: disable the built-in Code extension from StarterKit and register it
separately with a corrected regex that uses a lookbehind assertion
(?:^|(?<=[^`])) instead of a capture group. The lookbehind asserts the
preceding character without including it in the match, so markInputRule
only deletes the backtick delimiters.

Functionally tested on Firefox and Chrome.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

apps/client/src/features/editor/extensions/extensions.ts

@@ -1,4 +1,6 @@
+import { markInputRule } from "@tiptap/core";
 import { StarterKit } from "@tiptap/starter-kit";
+import { Code } from "@tiptap/extension-code";
 import { TextAlign } from "@tiptap/extension-text-align";
 import { TaskList, TaskItem } from "@tiptap/extension-list";
 import { Placeholder, CharacterCount } from "@tiptap/extensions";
@@ -113,10 +115,24 @@ export const mainExtensions = [
       color: "#70CFF8",
     },
     codeBlock: false,
-    code: {
+    code: false,
+  }),
+  // Override TipTap's Code extension to fix the inline code input rule.
+  // The upstream regex /(^|[^`])`([^`]+)`(?!`)$/ captures the character
+  // before the opening backtick as part of the match, causing markInputRule
+  // to delete it. Using a lookbehind avoids including it in the match.
+  Code.configure({
     HTMLAttributes: {
       spellcheck: false,
     },
+  }).extend({
+    addInputRules() {
+      return [
+        markInputRule({
+          find: /(?:^|(?<=[^`]))`([^`]+)`(?!`)$/,
+          type: this.type,
+        }),
+      ];
     },
   }),
   SharedStorage,