fix(canvas): stop self-closing embed from starting a greedy block match

extractCanvasShortcodes used a block regex whose open-tag attrs group
allowed a trailing slash, so a self-closing "[embed ... /]" open tag
could start a block embed match. The block pattern then greedily
swallowed all visible text up to a later stray "[/embed]", deleting that
text from channel delivery.

Anchor the block open-tag group so the attrs cannot end with a slash,
which prevents a self-closing open tag from initiating a block match.
The self-closing shortcode is now matched only by selfClosingRe and the
surrounding visible text is preserved.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/chat/canvas-render.test.ts

@@ -0,0 +1,38 @@
+// Canvas-render tests cover [embed] shortcode extraction and text stripping.
+import { describe, expect, it } from "vitest";
+import { extractCanvasShortcodes } from "./canvas-render.ts";
+
+describe("extractCanvasShortcodes", () => {
+  it("does not let a self-closing embed start a greedy block match", () => {
+    // Regression: the block regex used to greedily swallow the span from a
+    // self-closing "[embed ... /]" open tag up to a later stray "[/embed]",
+    // deleting the visible text in between (" keep me ") from channel delivery.
+    const input = '[embed url="https://a.com" /] keep me [/embed]';
+    const { text, previews } = extractCanvasShortcodes(input);
+
+    expect(previews).toHaveLength(1);
+    expect(previews[0]?.url).toBe("https://a.com");
+    // The visible text between the self-closing embed and the stray close
+    // marker must be preserved, not silently stripped.
+    expect(text).toContain("keep me");
+    expect(text).toBe("keep me [/embed]");
+  });
+
+  it("still extracts a normal block embed and strips only the shortcode span", () => {
+    const input = 'before [embed ref="doc1"] hi [/embed] after';
+    const { text, previews } = extractCanvasShortcodes(input);
+
+    expect(previews).toHaveLength(1);
+    expect(previews[0]?.viewId).toBe("doc1");
+    expect(text).toBe("before  after");
+  });
+
+  it("still extracts a plain self-closing embed and keeps surrounding text", () => {
+    const input = 'see [embed url="https://b.com" /] end';
+    const { text, previews } = extractCanvasShortcodes(input);
+
+    expect(previews).toHaveLength(1);
+    expect(previews[0]?.url).toBe("https://b.com");
+    expect(text).toBe("see  end");
+  });
+});

src/chat/canvas-render.ts

@@ -203,7 +203,10 @@ export function extractCanvasShortcodes(text: string | undefined): {
     attrs: Record<string, string>;
     body?: string;
   }> = [];
-  const blockRe = /\[embed\s+([^\]]*?)\]([\s\S]*?)\[\/embed\]/gi;
+  // Exclude a self-closing open tag ("[embed ... /]") from starting a block
+  // match by requiring the attrs group not to end with a slash; otherwise the
+  // block regex greedily swallows visible text up to a later stray [/embed].
+  const blockRe = /\[embed\s+([^\]]*?[^\]/]|)\]([\s\S]*?)\[\/embed\]/gi;
   const selfClosingRe = /\[embed\s+([^\]]*?)\/\]/gi;
   for (const re of [blockRe, selfClosingRe]) {
     let match: RegExpExecArray | null;