It's the (external) html editor that steals the focus. I was sure this 
had been fixed in newer releases, but maybe not. You can trying using 
a different wysiwyg editor, but this isn't anything that could be 
fixed in Horde.

This one's just an irritant - when composing messages in HTML 
(options-mail-message composition, "Compose messages with an HTML GUI 
by default"), when one clicks on "new message" and the new message 
window appears - and I start keying in the "to" email addresses, as 
the screen refreshes (presumably shifting to HTML), the cursor is 
suddenly repositioned into the body of the email.  This attempts 
expansion of the partially keyed email address (which usually fails 
because not enough has been keyed) and one has to reposition the 
cursor to complete entry of the "to" and "subject" fields.   
Workaround:  turn HTML off or don't start keying anything until its 
refreshed, then manually move cursor to the "to" field.