[Edit]

JavaScript - simple vertical splitter component in Vanilla JS

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<!doctype html>
<html>
<head>
  <style>

    div.splitter {
        border: 1px solid #e4e4e4;
        width: 300px; height: 200px;
        display: flex;
        flex-direction: column;
    }
    
    div.panel {
        flex: auto;
        max-height: calc(100% - 6px);
        overflow: auto;
    }

    div.resizer {
        position: relative;
        border: 1px #e4e4e4;
        border-style: solid none;
        background: rgb(228, 228, 228, 20%) url('data:image/svg+xml,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%2230.267%22%20height%3D%226.267%22%20viewBox%3D%220%200%208.006%201.658%22%3E%3Cpath%20fill%3D%22%23cfcfcf%22%20d%3D%22M8.008%200v.6h-.6V0Zm0%201.058v.6h-.6v-.6zM6.95%200v.6h-.6V0Zm0%201.058v.6h-.6v-.6zM5.892%200v.6h-.6V0Zm0%201.058v.6h-.6v-.6zM4.832%200v.6h-.6V0Zm0%201.058v.6h-.6v-.6zM3.776%200v.6h-.6V0Zm0%201.058v.6h-.6v-.6zM2.717%200v.6h-.6V0Zm0%201.058v.6h-.6v-.6zM1.657%200v.6h-.6V0Zm0%201.058v.6h-.6v-.6zM.6%200v.6H0V0Zm0%201.058v.6H0v-.6z%22%2F%3E%3C%2Fsvg%3E') no-repeat center / 18px;
        flex: 0 0 6px;
        cursor: row-resize;  /* OR:  n-resize */
        /* the below styles disable selection for resizer element           */
        -webkit-touch-callout: none;  /* iOS Safari                         */
          -webkit-user-select: none;  /* Safari                             */
           -khtml-user-select: none;  /* Konqueror HTML                     */
             -moz-user-select: none;  /* Firefox in the past (old versions) */
              -ms-user-select: none;  /* Internet Explorer (>=10) / Edge    */
                  user-select: none;  /* Currently supported:               */
                                      /* Chrome, Opera and Firefox          */
    }

    div.resizer:hover::after {
        position: absolute;
        top: -6px; right: 0; bottom: -6px; left: 0;
        background: rgb(228, 228, 228, 30%);
        display: block;
        content: ''
    }

  </style>
  <script type="text/javascript">

    function computeMargins(element) {
		var style = element.currentStyle || window.getComputedStyle(element);
        return {
            marginTop: parseInt(style.marginTop),
            marginBottom: parseInt(style.marginBottom)
        };
    }

    function prepareResizer(master, resizer) {
        var removed = false;
        var dragging = false;
        var position;
        var style = master.style;
        var handleMouseDown = function(event) {
            var margins = computeMargins(master);
            position = margins.marginTop + margins.marginBottom + master.offsetHeight - event.clientY;
            dragging = true;
        };
        var handleMouseUp = function(event) {
            dragging = false;
        };
        var handleMouseMove = function(event) {
            if (dragging) {
                var y = position + event.clientY;
                style.flex = `0 0 ${y}px`;
            }
        };
        var handleTouchStart = function(event) {
            var touches = event.touches;
            if (touches.length === 1) {
                event.preventDefault();
                var touch = touches[0];
                handleMouseDown({
                    clientX: touch.clientX,
                    clientY: touch.clientY
                });
            }
        };
        var handleTouchEnd = function(event) {
            var touches = event.touches;
            if (touches.length === 0) {
                handleMouseUp({});
            }
        };
        var handleTouchMove = function(event) {
           if (dragging) {
                event.preventDefault();
                var touches = event.touches;
                if (touches.length === 1) {
                    var touch = touches[0];
                    handleMouseMove({
                        clientX: touch.clientX,
                        clientY: touch.clientY
                    });
                }
            }
        };
        resizer.addEventListener('mousedown', handleMouseDown);
        resizer.addEventListener('touchstart', handleTouchStart);
        window.addEventListener('mouseup', handleMouseUp);
        window.addEventListener('touchend', handleTouchEnd);
        window.addEventListener('mousemove', handleMouseMove);
        window.addEventListener('touchmove', handleTouchMove);
        return function() {
            if (removed) {
                return;
            }
            resizer.removeEventListener('mousedown', handleMouseDown);
            resizer.removeEventListener('touchstart', handleTouchStart);
            window.removeEventListener('mouseup', handleMouseUp);
            window.removeEventListener('touchmove', handleTouchEnd);
            window.removeEventListener('mousemove', handleMouseMove);
            window.removeEventListener('touchend', handleTouchMove);
            removed = true;
        };
    }

  </script>
</head>
<body>
  <div class="splitter">
    <div id="master" class="panel">Panel 1</div>
    <div id="resizer" class="resizer"></div>
    <div class="panel">Panel 2</div>
  </div>
  <script>

      var master = document.querySelector('#master');
      var resizer = document.querySelector('#resizer');

      prepareResizer(master, resizer);

  </script>
</body>
</html>

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