The ResponsiveSidebar is a HelicopterParentChild with one parent and two children:
 
 * `<page-root>` This is the invisible container for the entire structure.
 * `<page-menu>` This is area where the menu is.
 * `<page-content>` This is the area where the content is.
 
The `<page-root>` keeps track of its size using media queries (or alternatively AutoLayoutAttributes).
The trigger points for when the `<page-root>` alters its behavior are changed using CSS `::slotted(...)`
selectors that targets the `<page-menu>` and `<page-content>` elements directly.
The `<page-root>` hides all the elements that are not <page-menu>` and `<page-content>`, although
this can be overridden from the lightDOM.

When the elements are displayed, then the 

```html
<template>
  <style>
    /*mobile by default*/
    ::slotted(*) {
      display: none;
    }
    .sidebar {
      margin: 0;
      padding: 0;
      width: 200px;
      background-color: #f1f1f1;
      position: fixed;
      height: 100%;
      overflow: auto;
    }
    ::slotted(page-menu),
    ::slotted(page-content) {
      display: block;
      float: left;
      width: 100%;
    }
    ::slotted(page-menu) {
      height: var(--page-menu-height, 20px);
    }
    ::slotted(page-content) {
    }
    
    /*desktop*/
    @media screen and (min-width: 600px) {   /*the 600px will be replaced by another number when an attribute change*/
      ::slotted(*) {
        display: none;
      }
      ::slotted(page-menu),
      ::slotted(page-secondary),
      ::slotted(page-content) {
        display: block;
        float: left;
        min-height: 100%;
      }
      ::slotted(page-menu) {
        height: var(--page-menu-width, 20px);
      }
      ::slotted(page-content) {
      }
    } 
  </style>
</template>

<script >

class PageRoot extends HTML {
  constructor(){
    super();
    this.attachShadow({mode: "open"});
    const template = document.querySelector("template").content;
    this.shadowRoot.appendChild(template.cloneNode(true));
  }
}
class PageMenu extends HTML {
  
}
class PageContent extends HTML {
  
}

customElements.define("page-root", PageRoot);
customElements.define("page-menu", PageMenu);
customElements.define("page-content", PageContent);
</script>

<style>
page-menu{
  background-color: lightblue;
}
page-content {
  background-color: pink;
}
</style>

<page-root>
  <page-menu>menu</page-menu>
  <page-content>content</page-content>
</page-root>
```