The gap between the window header and the tabs in popup

There is a gap between the window header and the row of tabs in the popup window.

Steps to reproduce:

1. Find a Host with a lot of items (like default Zabbix server)
2. Go to Items of the Host
3. Select all the Items
4. Press Mass update button (scrollbar should appear in the popup)
5. Scroll down the list

Result:

The gap is present between the window header and the row of tabs: The_gap_mass.gif.

Expected:
No gap between.