With T183375, undeletion no longer sets the parent ids for the undeleted revisions to the "expected" one given by the previous revision by id (usually, but not always, also the previous revision by timestamp), but instead to the ar_parent_id values. For example, look at the [[https://en.wikipedia.org/w/index.php?title=Ernest_Madu&action=history|history of Ernest Madu on Wikipedia]], and you'll see that [[https://en.wikipedia.org/w/index.php?title=Ernest_Madu&oldid=820864133|revision 820864133]] has parent id 0 (with an "N" mark in the [[https://en.wikipedia.org/wiki/Special:Contributions/Andrewkazimi|user's contribs]]) instead of [[https://en.wikipedia.org/w/index.php?title=Ernest_Madu&oldid=820863993|820863993]]. While T183375 may have been a good idea, there are situations (such as selective undeletion or history splits) where it would be better to reset the parent ids like they had always been.
Add a new checkbox (unchecked by default) saying "Reset parent ids" or something similar that, if checked, will fallback to the old behavior of resetting the parent revisions. If unchecked, ar_parent_id will be used as the rev_parent_id for the undeleted revisions, which is the current apparent behavior per T183375.
**Advantages (pros) of resetting parent ids**
* Never makes revisions with broken parent revisions (T186280; useful when removing unwanted edits with selective undeletion, although using revision deletion would be even better)
* Always treats the oldest revision as a page creation (useful when splitting histories)
* Allows fixing incorrect cases (e.g. [[https://en.wikipedia.org/wiki/Template_talk:Db-g1/Archive_1|Template talk:Db-g1/Archive 1]] or [[https://en.wikipedia.org/wiki/Gema_Switzerland|Gema Switzerland]] on Wikipedia; usually caused by imports done in 2015 or earlier or undeletions of revisions deleted in pre-1.5 versions of MediaWiki; see also T38976)
* May be used to correct previous selective undeletions that should have been full ones (otherwise, we might get 2 revisions with the same parent revision)
* May also be used repeatedly to correct the first disadvantage below, by undeleting only the latest group of revisions and then moving backward to earlier groups
**Disadvantages (cons) of resetting parent ids**
* May cause creation edits to suddenly become treated as non-creation edits if a deleted page has been recreated
* May cause minor edits to no longer appear minor in terms of the red negative or green positive "change in size" number in the history
* May cause null revisions resulting from imports to have rev_parent_id set to the latest revision imported at the time instead of the immediately preceding revision by timestamp with the same text
In general, to determine whether the proposed checkbox should be used or not, ask yourself: Does resetting the parent revisions improve the structure of the page's history?. If the answer is "yes", then use the checkbox; otherwise don't. Theoretically, the checkbox may allow the parent revision for a given revision to be changed to any desired revision with a smaller id and earlier timestamp. But as long as the checkbox is not being used abusively, this shouldn't be a problem to worry about.