Using fields with docvalues (like `_doc`) for tie-breaking yields
significantly better performance than using `_uid`, which lacks
docvalues at the moment. The downside is that sorting by `_doc` by
default is not stable under all conditions, but better than no
tie-breaking at all.
The new setting `context:tieBreakingFields` enables the user to
customize the list of fields Kibana attempts to use for tie-breaking.
The first field from that list, that is sortable in the current index
pattern, will be used. It defaults to `_doc`, which should change to
`_seq_no` from version 6.0 on.
In addition to just showing a notification, errors that occur while
loading documents from the database will be stored as part of the
`loadingStatus` along with a reason code (if known). This is used to
display more nuanced and helpful error messages to the user.
The first such error message indicates a missing or invalid tiebreaker
field required for sorting the context.
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