The previous behavior, fairly rough, was simply considering all
Collections and IDs found as part of the root's hierarchy as to be
overridden.
The new behavior is based on a specific tag added to some ID usages.
Currently, these are only the links from a Collection to its Objects
and children Collections, and the link from an Object to its parent.
The goal of this huge reduction of the 'automatically overridden' set of
data in a hierarchy is to avoid creating liboverrides for 'utils' data,
e.g. a collection used as source of data by a GeometryNode generating
some parametric geometry (Blender Studio use-case: some form of LOD
handling).
In general, it means that only Collections and objects part of the
collection hierarchy of the root collection are now overridden.
This should not change in the typical recommended use-case so far (where
there is a clear root collection to the whole overridden hierarchy, and
it contains all related collections and objects).
But this should allow much less overhead (and viewport pollution, due to
the current principle that local (and therefore liboverridden) objects
have to be instanciated in a Scene) for more complex setups.
Note that the initial idea/change is fairly simple and easy to
implement, but it creates some additional complexity in the partial
resync code, since now some linked data may not be detected as needing
a liboverride anymore when processing a sub-hierarchy, while it would
require it when processing the whole hierarchy.
This required adding some more processing to the util functions which
define which ID should be overridden. It essentially creates a
'reference set' of all reference IDs that should be overridden when
the whole hierarchy is considered, and use this as additional input to
decide which ID to override when processing a sub-hierarchy for the
partial resync.
Pull Request: https://projects.blender.org/blender/blender/pulls/115853
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