[Svelte] 在隐藏源恢复之前,DragOverlay 会消失一帧

When the source content is hidden using `draggable.isDragSource`, the default drop animation ends with a brief blank frame: the overlay disappears before the source becomes visible again. The source element is already in the DOM and remains connected throughout the animation and cleanup. This reproduces with a single draggable, without Query, network requests, list updates, or application state changes.