Problem description: When adding a widget to a dashboard using a slow connection, if You click on the placeholder right after the widget configuration overlay dialogue closes, the time between closure of widget configuration overlay dialogue and appearance of the widget on the dashboard is enough to initiate creation of a second widget in the same place.
Afterwards, by slightly dispositioning the widget on top it is possible to position the bottom widget partially outside of dashboard area.

Steps to reproduce:

1. Create a dashboard
2. Make sure that You're using (or simulating) a slow internet connection
3. In the top left corner of the dashboard click on the placeholder to create a widget.
4. Configure any widget in the opened overlay dialogue and press Add.
5. Click in the same place on the dashboard (top left corner) right after the overlay dialogue has closed, but before the widget appeared on the dashboard.
6. In the opened overlay dialogue configure another widget and press Add button.
7. Wait until both widgets are added.
   Result 1: the 2nd widget is placed above the 1st one.
8. Attempt to change the positioning of both widgets by dragging the above widget a little downwards and then a little upwards.
   Result 2: The below widget is placed partially outside of dashboard area.

Expected: There shouldn't be a possibility for a user to place one widget above another.

Example: