UVE: Avoid Changing iframe URL in Headless Mode for Faster Content Updates

Parent Issue

#30943

Background

Currently, in headless mode, every time the client sends a new URL, the UVE reloads the iframe. However, modern frameworks, using our hooks, are capable of handling content updates efficiently without the need for a full iframe reload. This results in an unnecessary double reload, slowing down the process and reducing performance.

Overview

When operating in headless mode, we should avoid modifying the iframe URL upon receiving a new URL from the client. Instead, the framework will handle content updates internally, and the UVE should rely on postMessage to fetch and update the data. This approach will eliminate redundant reloads, ensuring a faster and smoother user experience.

Video

https://github.com/user-attachments/assets/fff6d55c-c79b-4eed-82e0-4628529e8172

Task

1. Prevent iframe URL Mutation: Ensure the iframe URL remains unchanged when in headless mode.
2. Implement Data Fetching via postMessage: Update the data dynamically through postMessage without reloading the iframe.
3. Optimize Framework Communication: Ensure the framework efficiently manages content updates without requiring a full iframe reload.
4. Document Changes: Clearly document this behavior in the UVE documentation to inform developers of the new optimization.

Proposed Objective

Core Features

Proposed Priority

Priority 3 - Average

Acceptance Criteria

```[tasklist]

Acceptance Criteria

• The iframe URL remains static in headless mode.
• Data updates occur through postMessage without triggering a full iframe reload.
• Performance improvements are observed by avoiding double reloads.
• Documentation is updated to reflect these changes and guide developers.
  \`

Start by locating the UVE headless-mode handling for iframe URL updates and the existing postMessage communication entry points. Trace how a new client URL currently triggers reloads, then verify that the iframe remains static, updates arrive through postMessage, and the UVE documentation describes the behavior. No specific files or tests are named in the issue.