There is a typo in skins/default/templates/compose.html in relation to the WRAP attribute:

<roundcube:object name="composeBody" id="compose-body" form="form" cols="80" rows="20" warp="virtual" tabindex="7" />

WRAP is misspelled as WARP. Changing it to WRAP should fix this problem (at least it did for me).

Cheers, -Roger