Learndash has a focus mode which provides students with a distraction-free page layout. Learndash implements this by hiding other elements on the page, once the page has loaded. However, in the Divi theme a custom header built with the Divi Theme Builder will typically start to display before the page has fully loaded, and can flash up briefly while focus mode loads. Here's how to prevent it.

Hide the Header on Focus Mode using Divi LearnDash Kit

Divi LearnDash Kit is my plugin for improving the integration of Divi and LearnDash. When installed, Divi LearnDash Kit automatically fixes several compatibility issues, including the issue of the header flashing up in focus mode. 

The focus mode header flicker issue is addressed in Divi LearnDash Kit v1.5.2 and upwards.

Hide the Header on Focus Mode using CSS

Using CSS, we can hide the header sooner when focus mode is active, preventing it from flashing up while focus mode loads. Here's the CSS: