Why does the menu block overlap part of the first screen?

Please look at the menu positioning settings (Block settings → Main settings → Menu position behaviour).

If "Fixed on scroll" is selected, the menu will overlap the first block in any case. You can solve this problem by adding an extra padding equal to the height of the menu at the beginning of the page. You can use the DV05 block to do this.   

Instead of the DV05 block, you can set the top padding in the settings of the first block as well.