About Flex headers for Notebook components
A Flex Layout is added to the form's list of Form Components and the notebook tabs are grouped into a single tab and displayed as a dropdown selection of tabs when the notebook's property Tabs As Dropdown is set to true.
The Flex Layout is fully designable, where regions can be added and each region's settings can be changed. All regions, except for the first one, which contains the dropdown selection for the grouped notebook tabs, can have controls added to them.
The Flex Layout, along with all of its design settings and contents, is kept within the form definition if the Tabs As Dropdown property is set to false. The Flex Layout remains hidden and has no impact on the form. Additionally, the Flex Layout re-appears exactly as it was when the Tabs As Dropdown is set to true.