The tooltip while dragging a collection in Scene mode in the Outliner
was always "Link inside Collection" even if the action performed was
different. This was because the collection_drop_init set the
from_collection always to NULL if the Outliner display mode was
currently set to Scene.
Commit that introduced this issue:
rB0f54c3a9b75be8f8db9022fb0aeb0f8d0d4f0299
The fix removes the check of the display mode and only sets the
from_collection to NULL if the ctrl (linking) key is held.