@iotivedo @sixthsense The issue is caused by the Widescreen layout, enabled by default. Change to Desktop layout in user settings and the display will be normal.
To change this as a default value in roundcube settings, edit the file
nano /usr/local/cwpsrv/var/services/roundcube/config/defaults.inc.php
Modify the following variable from widescreen to desktop:
// Interface layout. Default: 'widescreen'.
// 'widescreen' - three columns
// 'desktop' - two columns, preview on bottom
// 'list' - two columns, no preview
$config['layout'] = 'desktop';