Incremental rendering re-animates the full response when main content contains a file reference

• Copilot Chat Extension Version: built-in GitHub Copilot 0.66.0
• VS Code Version: 1.138.0 (arm64, commit 7debcd0e2acdea1c52de81bf9ee1620444407dda)
• OS Version: macOS 26.5.2 (build 25F84), Apple Silicon
• Feature: Streaming chat response rendering (main assistant content)
• Selected model: Not yet isolated; the trigger appears to be rendered content rather than a specific model
• Logs: No error observed; this is a visual rendering/animation issue

Relevant settings

This was observed with the following experimental settings:

{
  "chat.experimental.incrementalRendering.enabled": true,
  "chat.experimental.incrementalRendering.animationStyle": "blur",
  "chat.experimental.incrementalRendering.buffering": "off"
}

In the Settings UI, these are:

• Chat > Experimental > Incremental Rendering: Enabled
• Chat > Experimental > Incremental Rendering: Animation Style (blur)
• Chat > Experimental > Incremental Rendering: Buffering (off)

Other animation styles and buffering modes have not yet been isolated.

Steps to Reproduce

1. Enable Incremental Rendering and use the settings shown above.
2. Start a chat response that continues streaming after it emits a workspace file path, a file URL, or an inline file reference in the main assistant response body.
3. Observe the already-rendered chat content after the path/URL is converted into or rendered as a file reference.

Expected Behavior

Only newly appended content should receive the incremental-rendering animation. Content that has already been rendered should remain stable, and linkifying/rendering a file reference should not restart the animation for the full response.

Actual Behavior

When a file path or file URL is rendered as a reference in the main chat content, the full incremental-rendering animation restarts. As more content streams in, a response containing file references can appear to keep re-rendering/re-animating continuously instead of settling.

The location of the reference matters:

• Reproduces when the file path/reference appears in the main chat content.
• Has not been observed when the path/reference appears only inside a collapsible section (for example, a thinking/tool-details region).

Additional Context

#319248 reports a similar symptom for a completed Mermaid block during streaming, but that issue is Mermaid-specific and does not cover file references or the experimental Incremental Rendering configuration above. This report is specifically about file-reference rendering in the main assistant body.

The Incremental Rendering test plan is tracked in #311090.