Steps to reproduce:

1. Make the browser view small, e.g., half of your regular screen size in order for Graph or Pie chart widget popup to open over Zabbix main menu.
2. Add Graph or Pie chart widget to dashboard.
3. Click on downward arrow next to 'Add new data set' to access data set type menu dropdown.

Result:
Observe 'Add new data set' menu dropdown being stacked behind the widget popup and behind Zabbix main menu.

Expected:
'Add new data set' menu dropdown visible and layered on top of widget's popup.