Struct TabBarDragData 

pub struct TabBarDragData<T: 'static> {
    pub source_index: usize,
    pub source_bar_id: WidgetId,
    pub source_id: TabId,
    pub item: Option<T>,
}

Drag payload published by a tab header when the user starts dragging it.

Generic over the bar’s item type T so a TabBar<T> only ever downcasts (get_typed::<TabBarDragData<T>>()) a drag started by another TabBar<T> — a drag from a TabBar<OtherT> simply never matches, giving cross-bar transfer type-safety for free.

Two consumers:

• Intra-bar reorder: the bar’s own on_drop matches source_bar_id == self_id and uses source_index to drive move_item. item is unused on this path (and may be None).
• Cross-bar transfer: a different bar that opted in via accept_external_tabs takes item by value and hands it to its on_tab_received callback. item is Some only when the source bar opted in and the per-tab transferable predicate allows it (static tabs are excluded).

Fields

source_index: usize

Model index of the dragged tab in the source bar.

source_bar_id: WidgetId

Widget id of the source TabBar. The receiving bar compares it to its own id to tell an intra-bar reorder from a cross-bar transfer.

source_id: TabId

Stable id of the dragged tab — handed to the source bar’s on_transfer_out so the app can remove it by id.

item: Option<T>

A clone of the dragged item, carried for cross-bar transfer. None when the source bar didn’t opt into transfer or the tab is non-transferable (e.g. a static tab).