CSS Sticky Position

    Sticky starts normal, then sticks when scrolling.
    
    ## Example
    ```css
    .header {
      position: sticky;
      top: 0;
    }
    ```
    
    ## UI imagination
    Section title sticks while scrolling section.
    
    ## Real-world example
    - Table headers
    - Documentation menus
    
    ## Key requirement
    Parent must not have overflow hidden.