Selecting certain child datablocks also selects the parent (e.g.
selecting a pose bone selects the armature). The base was selected, but
the outliner tree element was left unselected. The subsequent selection
sync would then deselect the parent base because it was not flagged as
selected in the outliner.
This led to issues like T74332 where selecting a pose bone in the
outliner did not show drivers in the driver editor unless the armature
was explicitly added to the selection afterwards.
The solution is to also flag the outliner elements as selected when
selecting parent bases.
Differential Revision: https://developer.blender.org/D7029
fe7528ee91