Needs at least backporting of the fix to H5

This is very pity, because even if we are working on H6, we still need 
to support H5 for some time, until the next major version is stable.

This is what I was trying to say. ;-) In this case it works correctly.

I have to admit that in the case of moving a message out of a search 
folder while still meeting the search requirements, both possible 
behaviors may make sense, but may also be confusing. If I stick with 
my folder semantics, the current behavior is indeed correct, the 
message disappears from the current folder. It's still a bit confusing 
if the message re-appears on the next visit, but probably less 
confusing than the message instantly re-appearing.

That leaves that second example of dragging from virtual trash. In 
this case, it can be safely assumed that the user want's to undelete 
the message, just like he would do when dragging from a regular trash 
folder. What we need to do is not only to move the message to the drop 
target (if it wasn't already from this folder), but also to remove the 
\deleted flag. In this use case the folder metaphor not only works 
instantly, but also permanently.

Having separate "trash" and "virtual trash" options was completely 
unintuitive.  They are the same thing.  Virtual Trash is a *type* of 
trash mailbox; it's not a separate entity.

Unfortunately, this isn't something that is directly backportable 
since message viewing has changed a bunch in IMP 7.  Due to other time 
concerns (which I will hopefully be able to discuss very soon) I don't 
have the time to do the kind of involved backporting necessary for this.

Exactly my point.
Why?  Again... this is how every mailbox works... when you move a 
message out, it disappears.  And all search mailboxes are not 
refreshed until explicitly told to.  (There is a difference in 
semantics between search and non-search mailboxes, but that is OK 
because there is explicit visual indication - via the searchbar - of 
this difference).

I guess I'm confused what you think you should be seeing.  Moving a 
message out of the Virtual Trash should not cause that message to 
remain in the Virtual Trash.  I'm not sure what other behavior you are 
expecting.  (i.e. moving a message out of Virtual Trash to a different 
mailbox MUST cause the message to be initially removed from the 
virtual trash, since delivery to a mailbox could possibly result in 
the \deleted flag being removed, for example).

I found the old behavior with a checkbox to enable the virtual trash 
more intuitive. But I don't have a strong opinion about this either.
Needs to be backported too. Thanks!
It's not the same completely. Because if you move from a search 
mailbox to a folder so that it doesn't match the search criteria 
anymore, it works like a regular mailbox. And that's the more 
important point. If we use the folder semantics with search results, 
we need to use the folder semantics in user interaction too. Where we 
cannot guarantee this interaction behavior, we need to disable it. If 
we don't want to re-run the search immediately (which would be the 
correct behavior) because it is too performance hungry, we need to 
disable moving out of search folders. If we don't want to undelete 
message after moving from a virtual trash for whatever reasons (which 
would be the correct behavior too), we again need to disable moving.

Between IMP 4 and IMP 5.
Can't reproduce.
Fixed.
The current behavior is correct.  If you "move" a message out of 
Virtual Trash, it will disappear from virtual trash.  But since the 
message has a deleted flag, it will "re-appear" when the virtual trash 
is explicitly refreshed (i.e. the search is run again).  This is 
consistent semantics with every other search mailbox.

Changes have been made in Git (master):

commit cad459b54d0a2ea63a0c6cb27ef4164199408205
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Feb 25 00:03:34 2015 -0700

     Bug #13852: Fix display of delete-related options when using virtual trash

  imp/js/base.js                            |   28 +++++++++++++++-------------
  imp/lib/Ajax/Application/ListMessages.php |   14 ++++++++++++--
  imp/lib/Dynamic/Mailbox.php               |    6 ------
  3 files changed, 27 insertions(+), 21 deletions(-)

http://github.com/horde/horde/commit/cad459b54d0a2ea63a0c6cb27ef4164199408205

Now that's one of a hidden setting. I never use Virtual Trash, so when 
has it been moved there?
Now that I was able to enable Virtual Trash, it's broken though:
- You still see deleted messages in the regular mailboxes.
- You cannot undelete messages.
- You can drag messages from the VT to any folder while they stay deleted.

Trash Mailbox drop-down selection list in "Deleting and Moving 
Preferences" pref page.

Second option in that list (after "None") is "Use Virtual Trash".

What do you mean with "if you select it"?

I'm confused... if you disable Trash (i.e. Trash = None), why would 
Virtual Trash appear?

Virtual Trash should only appear if you explicitly select it (which I 
verify works).

After turning off the Trash folder, I don't get the Virtual Trash 
folder instead.