fix: support underscore italics in markdown renderer

packages/editor/demoPlan.ts

@@ -1,7 +1,7 @@
 export const DEMO_PLAN_CONTENT = `# Implementation Plan: Real-time Collaboration
 
 ## Overview
-Add real-time collaboration features to the editor using **[WebSocket API](https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API)** and *[operational transforms](https://en.wikipedia.org/wiki/Operational_transformation)*.
+Add real-time collaboration features to the editor using _**[WebSocket API](https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API)**_ and *[operational transforms](https://en.wikipedia.org/wiki/Operational_transformation)*.
 
 ## Phase 1: Infrastructure
 

packages/review-editor/utils/renderInlineMarkdown.test.tsx

@@ -0,0 +1,45 @@
+import { describe, it, expect } from "bun:test";
+import { renderInlineMarkdown } from "./renderInlineMarkdown";
+
+const isElement = (node: unknown): node is { type: string; props: { children?: unknown } } =>
+  typeof node === "object" && node !== null && "type" in node && "props" in node;
+
+const types = (nodes: unknown[]) => nodes.map((node) => (isElement(node) ? node.type : typeof node));
+
+describe("renderInlineMarkdown", () => {
+  it("renders underscore emphasis", () => {
+    const nodes = renderInlineMarkdown("_text_");
+    expect(nodes).toHaveLength(1);
+    expect(isElement(nodes[0])).toBe(true);
+    expect((nodes[0] as { type: string; props: { children?: unknown } }).type).toBe("em");
+    expect((nodes[0] as { type: string; props: { children?: unknown } }).props.children).toBe("text");
+  });
+
+  it("renders underscore emphasis in context", () => {
+    const nodes = renderInlineMarkdown("foo _bar_ baz");
+    expect(nodes.filter(isElement)).toHaveLength(1);
+    expect(types(nodes)).toContain("em");
+    expect((nodes.find(isElement) as { type: string; props: { children?: unknown } }).props.children).toBe("bar");
+    expect(nodes.filter((node) => typeof node === "string").join("")).toBe("foo  baz");
+  });
+
+  it("keeps intraword underscores literal", () => {
+    expect(renderInlineMarkdown("snake_case")).toEqual(["snake_case"]);
+    expect(renderInlineMarkdown("foo_bar_baz")).toEqual(["foo_bar_baz"]);
+    expect(renderInlineMarkdown("__init__")).toEqual(["__init__"]);
+  });
+
+  it("renders underscore emphasis after other inline tokens", () => {
+    const boldNodes = renderInlineMarkdown("**bold**_italic_");
+    expect(types(boldNodes)).toEqual(["strong", "em"]);
+    expect((boldNodes[1] as { type: string; props: { children?: unknown } }).props.children).toBe("italic");
+
+    const codeNodes = renderInlineMarkdown("`code`_italic_");
+    expect(types(codeNodes)).toEqual(["code", "em"]);
+    expect((codeNodes[1] as { type: string; props: { children?: unknown } }).props.children).toBe("italic");
+
+    const linkNodes = renderInlineMarkdown("[link](https://example.com)_italic_");
+    expect(types(linkNodes)).toEqual(["a", "em"]);
+    expect((linkNodes[1] as { type: string; props: { children?: unknown } }).props.children).toBe("italic");
+  });
+});

packages/review-editor/utils/renderInlineMarkdown.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 
 /**
- * Renders simple inline markdown: `code`, **bold**, *italic*, and
+ * Renders simple inline markdown: `code`, **bold**, *italic*, _italic_, and
  * fenced code blocks (```...```). Enough for review comments.
  */
 export function renderInlineMarkdown(text: string): React.ReactNode[] {
@@ -36,8 +36,8 @@ function renderInline(text: string, startKey: number): React.ReactNode[] {
   const nodes: React.ReactNode[] = [];
   let key = startKey;
 
-  // Match inline patterns: [text](url), `code`, **bold**, *italic*, bare URLs
-  const regex = /(\[([^\]]+)\]\((https?:\/\/[^)]+)\)|`[^`]+`|\*\*[^*]+\*\*|\*[^*]+\*|https?:\/\/[^\s<)\]]+)/g;
+  // Match inline patterns: [text](url), `code`, **bold**, *italic*, _italic_, bare URLs
+  const regex = /(\[([^\]]+)\]\((https?:\/\/[^)]+)\)|`[^`]+`|\*\*[^*]+\*\*|(?<!\w)_([^_\s](?:[\s\S]*?[^_\s])?)_(?!\w)|\*[^*]+\*|https?:\/\/[^\s<)\]]+)/g;
   let lastIndex = 0;
   let match: RegExpExecArray | null;
 
@@ -59,6 +59,9 @@ function renderInline(text: string, startKey: number): React.ReactNode[] {
       nodes.push(<code key={key++} className="inline-code">{token.slice(1, -1)}</code>);
     } else if (token.startsWith('**')) {
       nodes.push(<strong key={key++}>{token.slice(2, -2)}</strong>);
+    } else if (token.startsWith('_')) {
+      const italicText = match[4];
+      nodes.push(<em key={key++}>{italicText}</em>);
     } else if (token.startsWith('*')) {
       nodes.push(<em key={key++}>{token.slice(1, -1)}</em>);
     } else if (token.startsWith('http')) {

packages/ui/components/Viewer.tsx

@@ -763,27 +763,40 @@ const ImageLightbox: React.FC<{ src: string; alt: string; onClose: () => void }>
 };
 
 /**
- * Renders inline markdown: **bold**, *italic*, `code`, [links](url)
+ * Renders inline markdown: **bold**, *italic*, _italic_, `code`, [links](url)
  */
 const InlineMarkdown: React.FC<{ text: string; onOpenLinkedDoc?: (path: string) => void; imageBaseDir?: string; onImageClick?: (src: string, alt: string) => void }> = ({ text, onOpenLinkedDoc, imageBaseDir, onImageClick }) => {
   const parts: React.ReactNode[] = [];
   let remaining = text;
   let key = 0;
+  let previousChar = '';
 
   while (remaining.length > 0) {
     // Bold: **text** ([\s\S]+? allows matching across hard line breaks)
     let match = remaining.match(/^\*\*([\s\S]+?)\*\*/);
     if (match) {
       parts.push(<strong key={key++} className="font-semibold"><InlineMarkdown imageBaseDir={imageBaseDir} onImageClick={onImageClick} text={match[1]} onOpenLinkedDoc={onOpenLinkedDoc} /></strong>);
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
-    // Italic: *text*
+    // Italic: *text* or _text_ (avoid intraword underscores)
     match = remaining.match(/^\*([\s\S]+?)\*/);
     if (match) {
       parts.push(<em key={key++}><InlineMarkdown imageBaseDir={imageBaseDir} onImageClick={onImageClick} text={match[1]} onOpenLinkedDoc={onOpenLinkedDoc} /></em>);
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
+      continue;
+    }
+
+    match = !/\w/.test(previousChar)
+      ? remaining.match(/^_([^_\s](?:[\s\S]*?[^_\s])?)_(?!\w)/)
+      : null;
+    if (match) {
+      parts.push(<em key={key++}><InlineMarkdown imageBaseDir={imageBaseDir} onImageClick={onImageClick} text={match[1]} onOpenLinkedDoc={onOpenLinkedDoc} /></em>);
+      remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -796,6 +809,7 @@ const InlineMarkdown: React.FC<{ text: string; onOpenLinkedDoc?: (path: string)
         </code>
       );
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -830,6 +844,7 @@ const InlineMarkdown: React.FC<{ text: string; onOpenLinkedDoc?: (path: string)
         );
       }
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -850,6 +865,7 @@ const InlineMarkdown: React.FC<{ text: string; onOpenLinkedDoc?: (path: string)
         />
       );
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -905,6 +921,7 @@ const InlineMarkdown: React.FC<{ text: string; onOpenLinkedDoc?: (path: string)
         );
       }
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -917,17 +934,21 @@ const InlineMarkdown: React.FC<{ text: string; onOpenLinkedDoc?: (path: string)
       }
       parts.push(<br key={key++} />);
       remaining = remaining.slice(match.index + match[0].length);
+      previousChar = '\n';
       continue;
     }
 
     // Find next special character or consume one regular character
-    const nextSpecial = remaining.slice(1).search(/[\*`\[!]/);
+    const nextSpecial = remaining.slice(1).search(/[\*_`\[!]/);
     if (nextSpecial === -1) {
       parts.push(remaining);
+      previousChar = remaining[remaining.length - 1] || previousChar;
       break;
     } else {
-      parts.push(remaining.slice(0, nextSpecial + 1));
+      const plainText = remaining.slice(0, nextSpecial + 1);
+      parts.push(plainText);
       remaining = remaining.slice(nextSpecial + 1);
+      previousChar = plainText[plainText.length - 1] || previousChar;
     }
   }
 

packages/ui/components/plan-diff/PlanCleanDiffView.tsx

@@ -587,6 +587,7 @@ const InlineMarkdown: React.FC<{ text: string }> = ({ text }) => {
   const parts: React.ReactNode[] = [];
   let remaining = text;
   let key = 0;
+  let previousChar = "";
 
   while (remaining.length > 0) {
     // Bold: **text** ([\s\S]+? allows matching across hard line breaks)
@@ -598,14 +599,26 @@ const InlineMarkdown: React.FC<{ text: string }> = ({ text }) => {
         </strong>
       );
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
-    // Italic: *text*
+    // Italic: *text* or _text_ (avoid intraword underscores)
     match = remaining.match(/^\*([\s\S]+?)\*/);
     if (match) {
       parts.push(<em key={key++}><InlineMarkdown text={match[1]} /></em>);
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
+      continue;
+    }
+
+    match = !/\w/.test(previousChar)
+      ? remaining.match(/^_([^_\s](?:[\s\S]*?[^_\s])?)_(?!\w)/)
+      : null;
+    if (match) {
+      parts.push(<em key={key++}><InlineMarkdown text={match[1]} /></em>);
+      remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -620,6 +633,7 @@ const InlineMarkdown: React.FC<{ text: string }> = ({ text }) => {
         </code>
       );
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -637,6 +651,7 @@ const InlineMarkdown: React.FC<{ text: string }> = ({ text }) => {
         </a>
       );
       remaining = remaining.slice(match[0].length);
+      previousChar = match[0][match[0].length - 1] || previousChar;
       continue;
     }
 
@@ -649,16 +664,20 @@ const InlineMarkdown: React.FC<{ text: string }> = ({ text }) => {
       }
       parts.push(<br key={key++} />);
       remaining = remaining.slice(match.index + match[0].length);
+      previousChar = "\n";
       continue;
     }
 
-    const nextSpecial = remaining.slice(1).search(/[*`\[!]/);
+    const nextSpecial = remaining.slice(1).search(/[\*_`\[!]/);
     if (nextSpecial === -1) {
       parts.push(remaining);
+      previousChar = remaining[remaining.length - 1] || previousChar;
       break;
     } else {
-      parts.push(remaining.slice(0, nextSpecial + 1));
+      const plainText = remaining.slice(0, nextSpecial + 1);
+      parts.push(plainText);
       remaining = remaining.slice(nextSpecial + 1);
+      previousChar = plainText[plainText.length - 1] || previousChar;
     }
   }