Changes have been made in Git for this ticket:

Request #9203: Change cached imp view on dynamic_view pref change

http://git.horde.org/diff.php/imp/lib/Auth.php?rt=horde-git&r1=37a428f865c64b4605a428973d14487a477bc512&r2=fa2962a8d2508b82a41f06bb50ca4e08491cf9ad
http://git.horde.org/diff.php/imp/lib/Prefs/Ui.php?rt=horde-git&r1=7ffb5f05e6f151327942239f3ed24500e7ee5b35&r2=fa2962a8d2508b82a41f06bb50ca4e08491cf9ad

Good point. I guess we can't differentiate between whether the view 
has been selected on the login screen vs. by the preference. The 
latter was the case here, so I expected it to change the view if I 
change the preference. I think we should still change the current view 
if the preference is changed, even if the view has initially been 
picked on the login screen instead. This seems more intuitive to me.

This is expected.  The compose link goes to the IMP view for the 
current session.  Or else how would that work?  e.g. dynamic_view is 
on, but the user selects traditional view on the login page.

Actually, the setting doesn't apply at all until the user logs out.

When changing the dynamic_view preference, the Compose entry in the 
menu tree should be updated.