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HTML

              
                <div id="container" class="container">
    <div class="header-floating">This is a floating header</div>

    <h2 class="header">Header 1</h2>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>

    <h2 class="header">Header 2</h2>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 2...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>

    <h2 class="header">Header 3</h2>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 3...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>

    <h2 class="header">Header 4</h2>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 4...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>

    <h2 class="header">Header 5</h2>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 5...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>
    <p class="message">Test record 1...</p>

</div>

              
            
!

CSS

              
                body {
  font: 14px/16px Arial, Verdana;
  padding: 10px;
  background: #345a80;
}

.container {
  width: 550px;
  margin: 0 auto;
  background: #dddddd;
  position: relative;
  overflow-y: auto;
  height: 300px;
  padding: 10px;
}

.header, .header-floating {
  font-size: 24px;
  color: #345a80;
  font-weight: normal;
  padding: 10px 0;
  margin: 0;
}

.header-floating {
  background: rgba(255, 255, 255, 0.9);
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  display: none;
  padding-left: 10px;
  padding-right: 10px; 
}

.is-visible {
  display: block;
}
              
            
!

JS

              
                window.onload = function() {
    // Select floating element. There's only one such element on the page, so we're using querySelector method
    // that selects only one element.
    var floatingTitle = document.querySelector('.header-floating');
    // Select all title elements. There are several elements, so we're using querySelectorAll method that selects all
    // elements available within a page
    var titles = document.querySelectorAll('.header');
    // Select a scrollable container
    var scrollable = document.querySelector('.container');
    // Add a function to run on a scroll event
    scrollable.onscroll = function() {
        changeFloatingHeaderOnScroll(floatingTitle, titles);
    };

    function changeFloatingHeaderOnScroll(floatingTitle, titles) {
        // Stop the execution if no titles found
        if (!titles.length) {
            return;
        }
        // Height of the scrollable element
        var elementHeight =  scrollable.offsetHeight;
        // Current scrolling position (is equal to 0 when the scrolling is on top). Everything above is hidden.
        var scrollTopPosition =  scrollable.scrollTop;
        // The last visible point at the bottom of the element. Everything below is hidden.
        var scrollBottomPosition = scrollTopPosition + elementHeight;
        // Elements for headers above and below current scrolling screen. We'll make use of them in case of no visible
        // headers on current screen.
        var lastAbove, lastBelow;

        // Indicator if fixed title is set. If it is, the iteration will be stopped.
        var fixedTitleIsSet;
        for (var i = 0, len = titles.length; i < len; i++) {
            if (!fixedTitleIsSet) {
                var title = titles[i];
                // Determine if the title is visible at the moment of scrolling
                var titleIsVisible = (title.offsetTop + title.offsetHeight) >= scrollTopPosition && title.offsetTop <= scrollBottomPosition;
                // Determine the last above title (above the scrolling point) and last below elements
                if (title.offsetTop <= scrollTopPosition) {
                    lastAbove = title;
                }
                else if (!lastBelow && title.offsetTop <= scrollTopPosition) {
                    lastBelow = title;
                }

                // If at the top of the element, hide the floating title.
                if (!scrollTopPosition) {
                    floatingTitle.style.display = 'none';
                    return;
                }

                // Set top value as the current scroll position
                floatingTitle.style.top = scrollTopPosition + 'px';

                // Now based on the determined title data, check different variations

                // If the title visible and is the first title, no floating title is needed
                if (titleIsVisible && i === 0) {
                    floatingTitle.style.display = 'none';
                    fixedTitleIsSet = true;
                }

                // If visible and there's a hidden header above, set the previous invisible header
                if (titleIsVisible && i > 0) {
                    floatingTitle.style.display = 'block';
                    floatingTitle.innerHTML = titles[(i - 1)].innerHTML;
                    fixedTitleIsSet = true;
                }

                // If invisible and the only element, set is as a floating title
                else if (!titleIsVisible && titles.length === 1){
                    floatingTitle.style.display = 'block';
                    floatingTitle.innerHTML = titles[i].innerHTML;
                    fixedTitleIsSet = true;
                }

                // If nothing is visible... set the last element above.
                else if (!titleIsVisible && titles.length === (i + 1) && lastAbove) {
                    floatingTitle.style.display = 'block';
                    floatingTitle.innerHTML = lastAbove.innerHTML;
                    fixedTitleIsSet = true;
                }

            }
        }
    }
};
              
            
!
999px

Console