Expose drag preview ref and allow opting out of the custom drag preview

[TrevorBurnham]

Background

Originally proposed in #133 (now closed).

Today, Tree always mounts DragPreviewContainer, which renders either the user's renderDragPreview or the built-in DefaultDragPreview. Inside useDragHook, we unconditionally call preview(getEmptyImage()) to suppress the browser's native drag image so our overlay can take its place.

This has two downsides:

1. No way to use the browser's native drag preview. Consumers who want the plain, GPU-accelerated browser drag image (with no custom overlay) can't get it — we always hide the native image.
2. Stutter on Chrome/macOS. When the custom preview moves outside the drop target, Chrome delays drop events by ~250ms, causing a visible stutter. Using the native preview avoids this.

Additionally, consumers currently have no way to attach a drag preview element separately from the drag handle. React-dnd's useDrag exposes both a ref (drag source) and a preview connector, but only dragHandle is forwarded to NodeRenderer.

Proposed API

Two independent additions:

1. previewHandle on NodeRendererProps

Additive, non-breaking. Forwards the preview connector from useDrag so consumers can attach it to a specific DOM node (e.g. a wrapper element that excludes toolbars/badges).

type NodeRendererProps<T> = {
  // ...existing fields
  dragHandle?: (el: HTMLDivElement | null) => void;
  previewHandle?: (el: HTMLDivElement | null) => void;
};

2. Opt-out for the custom drag preview

Let consumers say "use the browser's native preview, don't render DragPreviewContainer, don't call getEmptyImage()." Open question on the exact shape — options:

• <Tree disableDragPreview /> — boolean prop, simple.
• <Tree renderDragPreview={null} /> — reuse the existing prop; undefined keeps today's default, null opts out.

Either preserves backward compatibility: existing users with no renderDragPreview continue to see DefaultDragPreview.

Out of scope / follow-up

getEmptyImage() comes from react-dnd-html5-backend. Now that Tree accepts a custom dndBackend (#326), calling getEmptyImage() with a non-HTML5 backend is a latent bug. Worth fixing alongside this, but not required.

Prior art

PR #133 implemented (1) and a version of (2), but changed the default behavior (flipped the fallback from DefaultDragPreview to native), which would be a breaking visual change for every consumer who hasn't passed renderDragPreview. Closing that PR in favor of this issue.