## Sticky Header
The `PacemStickyHeaderElement` eases the functionalities of a _material-design_-like header.
```html
<pacem-sticky-header visibility="quickreturn"></pacem-sticky-header>
```
> It automagically binds itself to the **ancestor _Shell_ element** and then coordinates the appearence of the actual header based on the user' scrolling behavior.
In **this page** a `PacemStickyHeaderElement` gives the shared header a "sticky" behavior.
## Properties
- `offset` (readonly, does emit): returns the current scrolling offset (type: `{ "top": number, "left": number }`).
- `visible` (readonly, does emit): returns whether the header is currently visible or not (`boolean`).
- `visibility` (does not emit): gets or sets the visibility behavior. Allowed values are `'persistent'` (default) and `'quickreturn'`.
- `threshold` (does not emit): `number` that defines the scroll amount that triggers the visibility (default `0`).
## CSS Classes
The _Shell_ gets decorated with specific CSS classes accordingly to the current sticky state:
- `sticky-header-top`: the document scroll amount is below or equal to the `threshold`.
- `sticky-header-scroll`: the document scroll amount is beyond the `threshold`.
- `sticky-header-fade`: the header is not visible or hiding (might happen when 'quickreturn' is set as `visibility`).
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