css

/*Split*/
      #flex-split {
        display: flex;
        flex-direction: column;

        background-color: #fffbf7;

        width: 100%;
        height: calc(100vh - 84px*2);

        overflow: hidden;

        /*<!--box-sizing: border-box;-->*/
        /*<!--min-height: 10px;-->*/
        /*<!--z-index: 2;-->*/
        /*<!--overflow-y:visible;-->*/
      }

      .split-item {
        flex-grow: 0;
        width: 100%;

        overflow-y: scroll;
        overflow-x: hidden;
      }

      #split-divider {
        width: 100%;
        min-height: 10px;
        max-height: 10px;

        display: block;
        cursor: pointer;
        /*background-color: #fffbf7;*/
        background-color: #e7dfd6;
        margin: 10px 0;
      }

      #split-divider:hover {
       background-color: #d3cbc2;
      }

html

<div id="flex-split">

                <!--Main view-->
                <section id="main-view" class="split-item">

<div id="split-divider"></div>

                  <!--Additional view-->
                  <section id="additional-view" class="split-item">

simplePanelSplit.dart

 bool dividerHandled = false;
  bool dividerOnHold = false;
  int mousePositionOnStart = 0;
  Future<Null> prepareDividerHandler(Element divider) async {
    divider.onMouseDown.listen((MouseEvent event) {
      dividerOnHold = true;
      mousePositionOnStart = event.screen.y;
    });

    divider.parent.onMouseUp.listen((MouseEvent event) {
      if (dividerOnHold) dividerOnHold = false;
    });

    divider.parent.onMouseMove.listen((MouseEvent event) {
      if (!dividerOnHold) return;
      event.preventDefault();

      Element mainView = divider.parent.querySelector('#main-view');
      Element additionalView = divider.parent.querySelector('#additional-view');

      int dif = event.screen.y - mousePositionOnStart;
      mousePositionOnStart = event.screen.y;

      int mainViewHeight;
      try {
        mainViewHeight =
            int.parse(mainView.getComputedStyle().height.replaceAll('px', ''));
      } catch (err) {
        mainViewHeight = 0;
      }

      int additionalViewHeight;
      try {
        additionalViewHeight = int.parse(
            additionalView.getComputedStyle().height.replaceAll('px', ''));
      } catch (err) {
        additionalViewHeight = 0;
      }

      mainView.style.height = "${mainViewHeight + dif}px";
      additionalView.style.height = "${additionalViewHeight - dif}px";
    });

    dividerHandled = true;
  }