padding into window impossible to delete

[darkomenx]

In context of creation of splash screen, I want to locate a progress bar at the bottom of the window.
My main container AND window have the same size of 250 of height.

BUT

I can view a "padding" into window (after the 250 pixels) that can't overflowed.
I don't understand what this appear and particularly delete this.

Example :

Can you help me to understand what's this and to delete this please ?

Thank you for your help.