devWindow.css

#app {
    width: 800px;
    height: 650px;
    background-color: antiquewhite;
}

.dev-window {
    display: flex;
    flex-direction: column;
    height: 100%;
}
#top-part, #visual-part {
    width: 100%;
    display: flex;
    flex-direction: row;
}
#tree, #settings {
    height: 100%;
}
.split {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}
.gutter {
    background-color: #eee;

    background-repeat: no-repeat;
    background-position: 50%;
}
.gutter.gutter-vertical {
    background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAB4AAAAFAQMAAABo7865AAAABlBMVEVHcEzMzMzyAv2sAAAAAXRSTlMAQObYZgAAABBJREFUeF5jOAMEEAIEEFwAn3kMwcB6I2AAAAAASUVORK5CYII=');
    cursor: row-resize;
}
.gutter.gutter-horizontal {
    background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAeCAYAAADkftS9AAAAIklEQVQoU2M4c+bMfxAGAgYYmwGrIIiDjrELjpo5aiZeMwF+yNnOs5KSvgAAAABJRU5ErkJggg==');
    cursor: col-resize;
}