list: Add drag and drop support to ListItem

[lijingrs]

Description

Add on_drag, on_drop, and on_drag_hover methods to ListItem that delegate to the underlying base Stateful<Div>.

These methods enable drag-and-drop reordering when ListItem is used inside a Tree or any custom list layout. Without them, consumers cannot attach drag/drop handlers because the base field is private and ListItem does not implement Deref<Target=Div>.

Use case

In Verve (an API co-development platform built with gpui-component), the project tree needs drag-and-drop reordering for API requests and folders. The tree() component requires its render callback to return ListItem, so consumers have no way to wrap the item in an outer div with drag handlers. These three methods solve this by forwarding to the internal base div:

tree(&state, move |ix, entry, _selected, _window, cx| {
    ListItem::new(ix)
        .child(/* row content */)
        .on_drag(drag_value, move |drag, _pos, _window, cx| {
            // Mark as dragging + return a drag-preview entity
            cx.new(|_| drag.clone())
        })
        .on_drag_hover(move |hovering, _window, cx| {
            // Highlight as a potential drop target
        })
        .on_drop(move |drag, _window, cx| {
            // Reorder: move source relative to target
        })
})

The method signatures mirror GPUI's native Div::on_drag / on_drop / on_hover APIs, so they are immediately familiar.

What each method does

• on_drag<T, W>(value, constructor) — registers a drag value of type T. When the user drags the row, constructor builds a drag-preview entity of type W: Render. Delegates to self.base.on_drag().
• on_drop<T>(listener) — called when a drag value of type T is dropped onto this row. Delegates to self.base.on_drop().
• on_drag_hover(listener) — called with true when the cursor enters the row during a drag, false when it leaves. Delegates to self.base.on_hover().

Break Changes

Fully additive. No existing API changes. Existing ListItem consumers continue to work unchanged.

How to Test

cargo run

The on_drag / on_drop / on_drag_hover methods can be tested in any story that uses ListItem inside a list. The methods delegate directly to the underlying Stateful<Div>, so they inherit all of GPUI's native drag-and-drop behavior.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo fmt --check.
• [N/A] Tested macOS, Windows and Linux platforms performance (if the change is platform-specific) — not platform specific.

AI Assistance Disclosure

🤖 Parts of this PR were generated with AI assistance and subsequently reviewed by a human to match the project's existing code style and patterns. Specifically:

• The three method signatures were modeled after the existing on_click / on_mouse_down / on_mouse_enter builder methods and GPUI's native Div::on_drag / on_drop / on_hover APIs.
• The doc comments follow the same style as the existing methods in list_item.rs.

[huacnlee]

Please at least add simple example for this API in to story.