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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

            
              <div g-component="SampleComponent">
  <div class="indicator">
    <div g-ref="indicatorItems" class="indicator-item">Section 1</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 2</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 3</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 4</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 5</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 6</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 7</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 8</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 9</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 10</div>
  </div>
  <div g-ref="items" class="section">Section 1</div>
  <div g-ref="items" class="section">Section 2</div>
  <div g-ref="items" class="section">Section 3</div>
  <div g-ref="items" class="section">Section 4</div>
  <div g-ref="items" class="section">Section 5</div>
  <div g-ref="items" class="section">Section 6</div>
  <div g-ref="items" class="section">Section 7</div>
  <div g-ref="items" class="section">Section 8</div>
  <div g-ref="items" class="section">Section 9</div>
  <div g-ref="items" class="section">Section 10</div>
</div>


            
          
!
            
              .section
  height 200px
  background #ddd
  border none
  display flex
  justify-content center
  align-items center
  font-size 30px
  
  &:nth-child(odd)
    background #eee
    
.indicator
  position fixed
  font-size 16px
  
  &-item    
    border none
    animation blink .3s 
    
    &.is-in-viewport
      animation blink2 .3s 
      color #0a0
      
@keyframes blink
  0%
    color #f00
    
  100%
    color #000
    
@keyframes blink2
  0%
    color #f00
    
  30%
    color #f00  
    
  100%
    color #0a0
  
            
          
!
            
              class SampleComponent extends Component {
    constructor(element) {
        super(element);

        this.ref = {
            items: [],
            indicatorItems: [],
        }
    }

    mount() {      
        this.scrollHandler = this.scroll.bind(this);
        this.resizeHandler = this.resize.bind(this);
        window.addEventListener('scroll', this.scrollHandler);
        window.addEventListener('resize', this.resizeHandler);

        this.setState({
            displaySections: {},
            borders: [],
        });

        this.resize();
    }

    resize(event) {
        // calculate item top/bottom borders
        const borders = this.ref.items.map(item => {
            const rect = item.getBoundingClientRect();
            return {
                top: rect.top + window.pageYOffset,
                bottom: rect.top + window.pageYOffset + item.offsetHeight
            }
        });

        this.setState({
            borders: borders,
        });
      
       this.scroll();
    }

    scroll() {
        let displaySections = {};
        this.state.borders.forEach((item, index) => {
            // is in viewport
            if (item.top < window.pageYOffset + window.innerHeight && item.bottom > window.pageYOffset) {
                displaySections[index] = true;
            } else {
                displaySections[index] = false;
            }
        });

        this.setState({
            displaySections: displaySections,
        });
    }

    stateChange(stateChanges) {
        if('displaySections' in stateChanges) {
            Object.keys(stateChanges.displaySections).forEach(key => {
                this.ref.indicatorItems[key].classList.toggle('is-in-viewport', stateChanges.displaySections[key]);
            });
        };
    }
}

loadComponents({ "SampleComponent": SampleComponent });
            
          
!
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