Content Script UI disappears immediately after appearing.

Describe the bug

I spent the whole day looking for a solution to this problem. The elements appeared but then immediately disappeared.

Previously, I used Plasmo and it worked fine in the same selector.

web: https://www.istockphoto.com/en/search/2/image-film?phrase=water%20drink

https://github.com/user-attachments/assets/f86b0f93-26ac-4df1-b2d7-664fe364154e

import "~/assets/tailwind.css";

import ReactDOM from "react-dom/client";

export default defineContentScript({
  matches: ["https://www.istockphoto.com/*"],
  cssInjectionMode: "ui",
  runAt: "document_end",
  async main(ctx) {
    const ui = await createShadowRootUi(ctx, {
      name: "button-analyze-on-article",
      position: "inline",
      append: "first",
      anchor: "[data-testid='galleryContainer']",
      onMount: (container) => {
        const app = document.createElement("div");
        container.append(app);
        const root = ReactDOM.createRoot(app);
        root.render(
          <button className="bg-blue-500 text-white p-2 rounded-md">
            Analyze
          </button>
        );
        return root;
      },
      onRemove: (root) => {
        root?.unmount();
      },
    });

    ui.autoMount();
  },
});

Reproduction

Steps to reproduce

run dev

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (16) x64 AMD Ryzen 7 7840HS w/ Radeon 780M Graphics     
    Memory: 3.43 GB / 31.19 GB
  Binaries:
    Node: 22.14.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.9.2 - C:\Program Files\nodejs\npm.CMD
    pnpm: 10.14.0 - ~\AppData\Local\pnpm\pnpm.CMD
    bun: 1.2.12 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (140.0.3485.54)
  npmPackages:
    wxt: ^0.20.11 => 0.20.11

Used Package Manager

bun

Validations

• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the supplied defineContentScript reproduction and the createShadowRootUi lifecycle, then run it with bun against the stated iStockphoto URL. Done means the mounted Analyze button remains visible after the content script UI appears; no repository file or test path is named in the report.