Description of Problem:

A vFolder whose contents change a great deal messes up message display. For
example, a common use case is to use a vFolder to collect unread messages,
so a criteria on the message selection is "status | is not | read"

When you have a message from such a vFolder displayed in a window, things
are fine until the vFolder changes. If the message leaves the vFolder, ie
because it's not unread any more and a background mailcheck redoes the
vFolder, THE MESSAGE DISPLAY WINDOW GOES BLANK. This is a bit silly, as the
underlying message hasn't gone away at all. It is still in its source
folder - just not in the vFolder. 

(delete + expunge underneath a displayed message seems to have this effect,
which admittedly makes more sense - if you've deleted it, then there is
every reason for the message to no longer be displayed)


Steps to reproduce the problem:
1. create a vFolder which is subject to dynamic conditions, ie "not read"
2. view a message
3. refresh the vFolders (mail check, Ctrl-E Expunge, etc) causing the
message to no longer be in the vFolder

Actual Results:

Message window goes blank!

Expected Results:

Message window, since it's standalone, should continue to display the
message - very disconcerting when you are in the midst of reading an email
and it suddenly goes blank.

How often does this happen? 

Quite a bit - enough to be more than a "oh, what was that, hmm" and be 
annoying from a usability perspective: "oh, dammit, that happened again"

AfC