As an Owner of a board, you are able to change the visibility settings of the header bar. Two settings in the Board Customization settings allow you to customize this experience.
Start with header hidden
This setting is unique to a display user and a personal user. With it toggled on, each time you enter that particular board (or refresh), the header bar will not show. Instead, there will be a chevron icon (visible when the cursor is hovering at the top of the window) that one may click to show the header again.
With it toggled off, each time you enter that board (or refresh), the header bar will show up at the top.
Disable header
This setting is unique to a display user and a personal user. With it toggled on, the header bar will be disabled with no chevron to access it again.
IMPORTANT NOTE: In order to access the header after disabling, you will need to access the board settings via the dashboard. Here, you can toggle off the "Disable header" setting.
To access the board settings through the dashboard, access the three-dot menu associated with your desired board. Click on "Board settings".