Daniel: The bug only occurs when running Unity (not gnome-shell, etc.), and happens precisely at the moment that the Unity launcher becomes a drop target (either un-hiding itself if it was hidden, or hiding apps that are not drop targets if it was not hidden).
Additionally, in Unity 2D, the launcher does not become a drop target when you start a drag operation, so the bug does not occur.
Daniel: The bug only occurs when running Unity (not gnome-shell, etc.), and happens precisely at the moment that the Unity launcher becomes a drop target (either un-hiding itself if it was hidden, or hiding apps that are not drop targets if it was not hidden).
Additionally, in Unity 2D, the launcher does not become a drop target when you start a drag operation, so the bug does not occur.