Blank space at top of Rise page

I've never seen anything like that. Other elements have their own class names like "block-text" or "side-navbar" or "lesson-header__row," so if you can find out what the class name is of that blank white space, that may go a ways toward identifying its source. A web item's class name can be found by right-clicking the bar when previewing the course and choosing Inspect from the context menu. That'll open up the DevTools window, showing what class Rise is assigning that block/section.