Is it possible to disable the message/preview pane in 6.x ?

[joelbaltazor]

I am trying to disable the message/preview pane in imp.
Running the latest imp from dev-FRAMEWORK_6_0
horde/imp dev-FRAMEWORK_6_0 def3afd

In horde5 We had an imp/config/prefs.local.php

$_prefs['show_preview']['value'] = 0;
$_prefs['show_preview']['locked'] = true;

I've also tried:

$_prefs['preview_pane']['value'] = 0;
$_prefs['preview_pane']['locked'] = true;

I searched the horde database table horde_prefs and do not find either of those (or anything preview or pane related) set for any user on my system.

Is there a different preference I should be using?

Thank you,
Joel