Think I figured this out.  The original/pivot rows were being stored 
at the page level rather than the view level - so switching between 
views with selected messages could cause incorrect selections.

Solution was to store original/pivot rows within a view's metadata.

Changes have been made in Git (master):

commit 331edcbbb5967eff37e6d18f95b6802616d389c4
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Nov 7 00:31:28 2012 -0700

     [mms] Fix shift-clicking in dynamic mailbox view when navigating 
to other mailboxes (Bug #11520).

  imp/docs/CHANGES   |    2 ++
  imp/js/dimpbase.js |   23 ++++++++++++++---------
  imp/package.xml    |    2 ++
  3 files changed, 18 insertions(+), 9 deletions(-)

http://git.horde.org/horde-git/-/commit/331edcbbb5967eff37e6d18f95b6802616d389c4

Changes have been made in Git (develop):

commit 5818bd18f879cf2a2dee5bc300c1d0314be788c5
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Oct 16 00:42:48 2012 -0600

     Bug #11520: Fix slice range

  imp/js/viewport.js |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/5818bd18f879cf2a2dee5bc300c1d0314be788c5

I haven't seen this in a few days either, it might be fixed.

Setting correct milestone.

Ping?

Setting IMP 6 milestone.  Downgrading priority since I cannot reproduce.

I cannot replicate your issue - at least with the incorrect amount of 
messages selected.  I *was* able to replicate some strange behavior 
where the shift-click message was not included in the eventual 
selection (the actual selection ended a few messages before the 
clicked message).  This has been fixed - not sure if this is related 
to what you are experiencing.

Changes have been made in Git (master):

commit 5818bd18f879cf2a2dee5bc300c1d0314be788c5
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Oct 16 00:42:48 2012 -0600

     Bug #11520: Fix slice range

  imp/js/viewport.js |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/5818bd18f879cf2a2dee5bc300c1d0314be788c5

It seems to work fine if all rows are cached in the browser already, 
so the additional viewPort requests might be the key.

High prio because I accidentally flagged and deleted most of one of my 
mailboxes without noticing.

If I click a message in a large mailbox, then click-scroll several 
pages further, so far that new slices are loaded from the backend, and 
then shift click another message, there are far too many messages 
selected. Like 12000 instead of 200.