Taking this. It looks like the Automatic animation from TableView is inserting the new section (This pushes the section header down the table), and then removes the section (animating the section header that was pushed down to the right. 

I'll check if there's a quick config we can change to improve this.

Sent https://chromium-review.googlesource.com/c/chromium/src/+/1155923 for review.

I think we can improve the animation a little bit with no risk by changing the animation type. Here's a video of what I'm proposing:
https://drive.google.com/open?id=1jf1n56P4apUCHjC6ZqZlLe0mCEORsIzG

I think its better than what we have now, but maybe can be improved more by playing around with performBatchTableViewUpdates, but that's risky and I wouldn't want to do that for M69 since it can lead to crashes.

+ Martijn, PTAL at the video in case you feel the new animation is not better.

The CL fixes some constraints that were breaking when animating, because of this I think this should remain a P1.

Thanks Sergio I think what you proposed is definitely better and acceptable for M69. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/eb5e1734c257ce47c84cdda7ed8c5b9ba126f622

commit eb5e1734c257ce47c84cdda7ed8c5b9ba126f622
Author: sczs <sczs@chromium.org>
Date: Tue Jul 31 18:05:08 2018

[ios] Improves ReadingList header insert/delete animation

- Changes ReadingListTableVC section rowAnimation to
UITableViewRowAnimationMiddle and UITableViewRowAnimationFade.

- Constraints were breaking while animating, this because of a
known bug that was fixed in other headers (crbug.com/854117).
In order to fix this all the padding constraints priority was
lowered.

Video:
https://drive.google.com/open?id=1jf1n56P4apUCHjC6ZqZlLe0mCEORsIzG

Bug:  868228 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I5580fe7377f16070fc43b1bc283754d4b00d60ef
Reviewed-on: https://chromium-review.googlesource.com/1155923
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579479}
[modify] https://crrev.com/eb5e1734c257ce47c84cdda7ed8c5b9ba126f622/ios/chrome/browser/ui/reading_list/reading_list_table_view_controller.mm
[modify] https://crrev.com/eb5e1734c257ce47c84cdda7ed8c5b9ba126f622/ios/chrome/browser/ui/table_view/cells/table_view_text_header_footer_item.mm

Verified in:

App Version: 70.0.3515.0 canary
Devices: 10.3.3, 11.4.1, 12.0 beta 6
iOS Versions: iPhone 7 Plus, iPhone 8 Plus, iPad Air

New Animation looks good for reading list.

Video:
https://drive.google.com/open?id=1n24IAPNAYjj2VmM1HztjHiDw-hIUSExy

This bug requires manual review: Less than 23 days to go before AppStore submit on M69
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), kariahda@(iOS), cindyb@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Approved.

This issue has been approved for a merge. Please merge the fix to any appropriate branches as soon as possible!

If all merges have been completed, please remove any remaining Merge-Approved labels from this issue.

Thanks for your time! To disable nags, add the Disable-Nags label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Please merge asap.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e518b1908673739700edba52c90fdc482970679c

commit e518b1908673739700edba52c90fdc482970679c
Author: sczs <sczs@chromium.org>
Date: Tue Aug 14 20:40:41 2018

[ios] Improves ReadingList header insert/delete animation

- Changes ReadingListTableVC section rowAnimation to
UITableViewRowAnimationMiddle and UITableViewRowAnimationFade.

- Constraints were breaking while animating, this because of a
known bug that was fixed in other headers (crbug.com/854117).
In order to fix this all the padding constraints priority was
lowered.

Video:
https://drive.google.com/open?id=1jf1n56P4apUCHjC6ZqZlLe0mCEORsIzG

TBR=sczs@chromium.org

(cherry picked from commit eb5e1734c257ce47c84cdda7ed8c5b9ba126f622)

Bug:  868228 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I5580fe7377f16070fc43b1bc283754d4b00d60ef
Reviewed-on: https://chromium-review.googlesource.com/1155923
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#579479}
Reviewed-on: https://chromium-review.googlesource.com/1175085
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/branch-heads/3497@{#630}
Cr-Branched-From: 271eaf50594eb818c9295dc78d364aea18c82ea8-refs/heads/master@{#576753}
[modify] https://crrev.com/e518b1908673739700edba52c90fdc482970679c/ios/chrome/browser/ui/reading_list/reading_list_table_view_controller.mm
[modify] https://crrev.com/e518b1908673739700edba52c90fdc482970679c/ios/chrome/browser/ui/table_view/cells/table_view_text_header_footer_item.mm

Issue verified 
Version: Chrome Beta 69.0.3497.53
Device: iPhone 5C, 8
iOS: 11.4, 12.0

Animation verified as per comment 10
https://drive.google.com/open?id=17Y9Juof-V4VeadasODv1pnknHMa5PPjD