Support CloneIndependentRoot feature

[JiuqingSong]

Today when we extract HTML from editor using ExportHTMLFast feature, we first clone a DOM tree, then pass it to plugins to do clean up. However, if plugin wants to set src of image, it will trigger an image downloading service call. This is a browser behavior.

In this change, I create a new experimental feature CloneIndependentRoot, when set, we will create an independent HTML document using document.implementation.createHTMLDocument(), then call importNode() to clone the DOM tree, this makes the cloned tree to be under this independent document context, so setting src of IMG won't really download it.

[Copilot]

Pull Request Overview

This PR introduces the CloneIndependentRoot experimental feature to prevent network requests when cloning DOM elements during HTML export. When enabled, the feature creates an independent HTML document context for cloned elements, avoiding browser behavior that would trigger image downloads when setting src attributes.

Key changes:

• Added CloneIndependentRoot experimental feature flag
• Modified DOM cloning logic to use independent document when feature is enabled
• Added comprehensive test coverage for the new cloning behavior

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/roosterjs-content-model-types/lib/editor/ExperimentalFeature.ts	Added CloneIndependentRoot feature flag definition
packages/roosterjs-content-model-core/lib/editor/core/DOMHelperImpl.ts	Implemented independent document cloning logic and updated constructor
packages/roosterjs-content-model-core/lib/editor/core/createEditorCore.ts	Integrated feature flag with DOMHelper creation
packages/roosterjs-content-model-core/test/editor/core/DOMHelperImplTest.ts	Added test coverage for independent root cloning
demo/scripts/controlsV2/sidePane/editorOptions/ExperimentalFeatures.tsx	Added UI control for the new feature
demo/scripts/controlsV2/sidePane/editorOptions/EditorOptionsPlugin.ts	Enabled feature in demo configuration

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}