A picture still being written draws nothing, not an <img> with no source

remend 1.4 (#204) rewrites a half-written image to `![alt](streamdown:incomplete-image)`
where 1.3 dropped it. DOMPurify strips that source, so the chat showed a broken
picture until the closing bracket arrived.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Jing-yilin

canvas/src/markdown.test.ts

@@ -15,6 +15,13 @@ describe("renderMarkdown", () => {
     expect(renderMarkdown("run `bun test")).toContain("<code>bun test</code>");
   });
 
+  it("draws no picture until its source has arrived whole", () => {
+    expect(renderMarkdown("see ![shot](https://x.dev/a")).not.toContain("<img");
+    expect(renderMarkdown("see ![shot](https://x.dev/a.png)")).toContain(
+      'src="https://x.dev/a.png"',
+    );
+  });
+
   it("is a table only once the delimiter row has arrived", () => {
     expect(renderMarkdown("| a | b |")).not.toContain("<table");
     expect(renderMarkdown("| a | b |\n| - | - |")).toContain("<table");

canvas/src/markdown.ts

@@ -9,7 +9,7 @@
  */
 import DOMPurify from "dompurify";
 import { Marked } from "marked";
-import remend from "remend";
+import remend, { INCOMPLETE_IMAGE_PLACEHOLDER } from "remend";
 
 // The agent links what it made as `file:///…`, which DOMPurify drops and an http page could not
 // follow anyway. Relative to the project's page, `file/<path>` is the project's server
@@ -21,6 +21,11 @@ const md = new Marked({
       token.href = `file${url.pathname}${url.search}${url.hash}`;
     }
   },
+  // A picture a delta cut open comes out of remend under a source only Streamdown draws, which
+  // DOMPurify drops, leaving an `<img>` with none. Nothing is drawn until the rest arrives.
+  renderer: {
+    image: ({ href }) => (href === INCOMPLETE_IMAGE_PLACEHOLDER ? "" : false),
+  },
 });
 
 export function renderMarkdown(text: string): string {