Changes have been made in Git (FRAMEWORK_5_2):

commit 01e2cf522ca32c73f7d333b5892adf83b102714d
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Thu, 18 Aug 2016 10:02:52 -0400

Bug: 14447 Fix delete permissions.

  M js/kronolith.js
  M templates/chunks/permissions.inc

https://github.com/horde/kronolith/commit/01e2cf522ca32c73f7d333b5892adf83b102714d

Indeed. I should have checked my email before finishing this :)

Changes have been made in Git (FRAMEWORK_5_2):

commit 0dc92609bd8a0d815eb25490fce35fdbfceb5d59
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Thu Aug 18 10:00:11 2016 -0400

     Bug: 14447 Fix delete permissions.

  kronolith/js/kronolith.js                  |  4 ++--
  kronolith/templates/chunks/permissions.inc | 20 ++++++++++----------
  2 files changed, 12 insertions(+), 12 deletions(-)

http://github.com/horde/horde/commit/0dc92609bd8a0d815eb25490fce35fdbfceb5d59

Probably needs a backport of e9fcc074a35d5b5d64ca13b85887bac29b8c18cf

Confirmed in FW_52 only, works as expected in master.

When giving another user permissions on a shared calendar, the 
"Delete" permissions are not correctly displayed, which in turn can 
lead to them being removed unexpectedly.

For instance, if you give some user X all but "Delegate" permissions, 
the appropriate rights are saved in the database with perm_{2,4,8,16} 
all set to "1" (assuming the standard case of MySQL and 
"sharesng"-style tables). However, when closing the overlay window and 
returning to it later, there is no check mark displayed in the 
"Delete" column for that user.

Suppose you now add another user Y and give them, say, just Show and 
Read permissions and click on Save (without touching anything else), 
this also leads to user X's Delete permissions being removed because 
X's "Delete" check mark had been removed where it should not have been.

This is happening in Kronolith only, not in the other apps that use a 
seperate browser window to manage rights on shares (Nag, Turba, Mnemo).