Qt's Drag attached property doesn't like it at all when it gets deleted during drag.
Even though we try very hard to keep the popup open, when you have multiple notification applets, their dialogs don't know each other but access a shared data engine. This means that, since there can only we one hovered dialog at a time, another notification applet might still dismiss the notification and have our dialog close.
By using a singleton NotificationHelper we 1.) decouple the Drag from the item and 2.) share the "is dragging" state between applets.
BUG: 388684
FIXED-IN: 5.12.0