Blender cannot handle bones with (near) zero length. Prior to this
commit such bones were deleted when exiting Armature Edit mode. Now
they are kept and elongated so that they are numerically stable (or at
least they should be, given the threshold to the length that was
already in place).
To avoid the elongation from impacting the position of child bones,
they are disconnected from the tiny bone.
Apart from that it's quite nice for users that Blender no longer
silently deletes bones, this is also useful for the USD importer, as
it can import bones and expect them to exist afterwards (see
#147048).
Note: this only impacts armatures with bones of length ≤ 0.000001
units.
Pull Request: https://projects.blender.org/blender/blender/pulls/147814
07ccb021d2