When inserting a new widget in the dashboard, the minimal "imaginary" rows are 2.

This setting in the filesystem is responsible:

grep DASHBOARD_WIDGET_MIN_ROWS /usr/share/zabbix/include/defines.inc.php

Out of box it prints:

define('DASHBOARD_WIDGET_MIN_ROWS',     2);

It would be good to have '1' with a stock installation to save screen space or..
allow users to customize the field globally right in the GUI.

The biggest use case can be a "Plain text" widget without a header: