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HTML 

              
                <div id="app" @mousemove="doDrag(null, $event)">
  <div id='particles'><span class='particle' v-for="index in 20"></span></div>
  <h1>Border Radius Generator</h1>
  <div id="simple-switch">
    <input id='simple-switch-checkbox' type="checkbox" v-model='simple' @change='updateCorners'>
    <label for='simple-switch-checkbox'>
       <span class='switch'></span>
    </label>
    <span>Simple Mode</span>
  </div>
  <div ref='container' id='border-control' :class='{simple: (simple == true)}' :style='{height: height + "px", width: width + "px"}'>
    <div class='controls-container'>
      <span v-for="(corner, index) in corners" :data-order='index' @mousedown="startDrag(index)" :style='[corner.direction == "horizontal" ? {left: corner.move} : {top: corner.move}]'></span>
    </div>
    <div class='border-demo' :style='radius'></div>
  </div>
  <div id='size'>
    <div>
      <span>Width:</span>
      <input type='number' min="200" max="700" v-model='width' @change='handleResize'>px
    </div>
    <div>
      <span>Height:</span>
      <input type='number' min="200" max="700" v-model='height' @change='handleResize'>px
    </div>
  </div>
  <div id='display-radius'>
    border-radius: {{radius["border-radius"]}}
  </div>


</div>
!

CSS 

              
                /* Reference 
https://codier.io/creation/rJJwBfvBm
https://gradienthunt.com/
*/
@import url('https://fonts.googleapis.com/css?family=Varela+Round');


*{
	box-sizing: border-box;
	font-family: 'Varela Round';
}

body
{
	margin: 0;
}
#app {
  width: 100vw;
  height: 100vh;
	display: flex;
	flex-direction: column;
	align-items: center;
  background:  #2c5364;
	color: white;
	
	h1
	{
		text-transform: uppercase;
		margin: 2em 0 0.2em 0;
		font-style: underline;
		color: white;
		text-shadow: 1px 1px 1px black;
	}
} 

#border-control {
  display: flex;
  width: 400px;
  height: 400px;
	position: relative;
	margin: 20px 0;
	z-index: 5;
	&:hover
	{
		.controls-container
		{
			opacity: 1;
		}
	}
	
	.border-demo
	{	
		position: absolute;
		top: 0;
		left: 0;
		height: 100%;
		width: 100%;
		box-sizing: border-box;
		overflow: hidden;
		
		&::after
		{ 
			position: absolute;
			top: -250%;
			left: -250%;
			height: 600%;
			width: 600%;
			background-image: linear-gradient( 68.7deg,  rgba(29,173,235,1) 13.2%, rgba(137,149,250,1) 29.8%, rgba(229,109,212,1) 48.9%, rgba(255,68,128,1) 68.2%, rgba(255,94,0,1) 86.4% );
				background-size: 400% 400%;
				content: "";
				animation: move-gradient 60s ease infinite reverse, rotate-gradient 40s ease infinite reverse;
		}
	}
	
	.controls-container
	{
		opacity: 0.5;
		border: 2px dashed white;
		width: 100%;
		height: 100%;
		left: 0;
		top: 0;
		position: absolute;
		box-sizing: border-box;
		z-index: 1;
		transition: all 0.3s;
	}
	&.simple
	{
		span[data-order='1'], span[data-order='2'], span[data-order='7'], span[data-order='6'] 
	 {
	 	display: none;
	 }
	}
  span {
    position: absolute;
    background-color: white;
    height: 15px;
    width: 15px;
		 transform: translate(-50%,-50%);
		 cursor: pointer;
		 z-index: 1;
		 
	 &[data-order='2'], &[data-order='3'] 
	 {
	 	top: 100%;
	 }
	 
	 &[data-order='5'], &[data-order='6'] 
	 {
		left: 100%;
	 }
  }
}

#display-radius
{
	font-size: 1.2em;
	padding: 1em;
	font-style: normal;
	background-color: rgba(255,255,255,0.1);
	border-radius: 3% / 30%;
	margin: 1em 0;
	transition: all 0.3s;
	
	&:hover
	{
		background-color: rgba(255,255,255,0.7);
		color: rgba(0,0,0,0.7);
	}
}
#size
{
	display: flex;
	min-width: 400px;
	margin: 1em 0;
	font-size: 1.2em;
	> div
	{
		width: 50%;
		padding: 0.2em 0;
		input
		{
			width: 50px;
			background-color: transparent;
			color: white;
			border: 0px solid;
			border-bottom: 1px solid white;
			text-align: center;
			outline: 0;
		}
	}
}
#simple-switch {
    display: flex;
    margin-bottom: 1.2em;
    color: rgba(180, 180, 180, 0.5);
    input {
      display: none;
      &:checked + label {
        background-color: #3498db;

        .switch {
          left: 55%;
        }
      }
    }

    label {
      width: 30px;
      height: 15px;
      border-radius: 10px;
      background-color: gray;
      display: inline-block;
      margin-right: 10px;
      position: relative;
      transition: all 0.3s;
      cursor: pointer;

      span {
        background-color: white;
        height: 80%;
        width: 40%;
        position: absolute;
        border-radius: 50%;
        left: 5%;
        top: 8%;
        transition: all 0.3s;
      }
    }
  }
	
	
	
	// Particls BG
	#particles
	{
height: 100%;
width: 100%;
position: absolute;
overflow: hidden;
pointer-events: none;

$total-particles: 20;
$max-size: 70;
$min-size: 30;
$colors: rgba(255,255,255,0.2), rgba(255,255,255,0.1), rgba(255,255,255,0.3), rgba(255,255,255,0.5), rgba(255,255,255,0.4), rgba(0,0,0,0.1), rgba(0,0,0,0.2), rgba(0,0,0,0.3), rgba(0,0,0,0.4), rgba(0,0,0,0.5);

@function randomNum($min, $max) {
  $rand: random();
  $randomNum: $min + floor($rand * (($max - $min) + 1));

  @return $randomNum;
}

/*.random(min, max)
  return floor(math(0, 'random') * (max - min + 1) + min)
*/
@function randomcolor()
{
  @return nth($colors, randomNum(1, 10));
}

.particle
{
  position: absolute;
  border-radius: 50%;
  opacity: 0.5;
  animation-iteration-count: infinite;
  animation-timing-function: linear;
  will-change: transform;

  @for $i from 1 through $total-particles
  {
    &:nth-child(#{$i})
    {
      $animation-speed: randomNum(30, 60) * 2s;
      $animation-delay: randomNum(0, 300) * -1s;
      $particle-color: randomcolor();
      $particle-size: randomNum($min-size, $max-size) * 1px;
      $translate-start-x: calc(100vw + #{$particle-size});
      $translate-start-y: 0;
      $translate-end-y: randomNum(0, 50) - 10 * 1vh;
      $translate-end-x: 0;

@at-root
{
      @keyframes particle-#{$i}
      {
        0%
        {
          transform: translate($translate-start-x, $translate-start-y);
        }
        100%
        {
          transform: translate($translate-end-x, $translate-end-y);
        }
      }
}

      transform: translateX($translate-start-x);
      animation-name: particle- + $i;
      animation-delay: $animation-delay;
      animation-duration: $animation-speed;
      @if ($i % 2 == 0)
      {
        animation-direction: reverse;
      } 
      background: $particle-color;
      height: $particle-size;
      width: $particle-size;
      top: (randomNum(0, 100) * 1%);
      left: -($particle-size);
  }
    
  }}
	}
	
	
@keyframes move-gradient {
	0% {
		background-position: 0% 50%
	}
	50% {
		background-position: 100% 50%
	}
	100% {
		background-position: 0% 50%
	}
}

@keyframes rotate-gradient {
	0% {
		transform: rotate(0);
	}
	100% {
		transform: rotate(1turn);
	}
}
!

JS 

              
                new Vue({
  el: "#app",
  data() {
    return {
      corners: [],
      cornersFull: [{
        amount: "20%",
        move: "20%",
        direction: "horizontal"
      }, {
        amount: "20%",
        move: "80%",
        direction: "horizontal"
      }, {
        amount: "20%",
        move: "80%",
        direction: "horizontal"
      }, {
        amount: "20%",
        move: "20%",
        direction: "horizontal"
      }, {
        amount: "20%",
        move: "20%",
        direction: "vertical"
      }, {
        amount: "20%",
        move: "20%",
        direction: "vertical"
      }, {
        amount: "20%",
        move: "80%",
        direction: "vertical"
      }, {
        amount: "20%",
        move: "80%",
        direction: "vertical"
      }],
      cornersSimple: [{
        amount: "40%",
        move: "40%",
        direction: "horizontal"
      }, {
        amount: "60%",
        move: "40%",
        direction: "horizontal"
      }, {
        amount: "60%",
        move: "40%",
        direction: "horizontal"
      }, {
        amount: "40%",
        move: "40%",
        direction: "horizontal"
      }, {
        amount: "40%",
        move: "40%",
        direction: "vertical"
      }, {
        amount: "40%",
        move: "40%",
        direction: "vertical"
      }, {
        amount: "60%",
        move: "40%",
        direction: "vertical"
      }, {
        amount: "60%",
        move: "40%",
        direction: "vertical"
      }],
      containerRect: {},
      dragging: false,
      moving: -1,
      radius: {
        "border-radius": ""
      },
      simple: true,
      height: 400,
      width: 400,
    }
  },
  methods: {
    handleResize: function() {
      var element = this.$refs.container;
      this.containerRect = element.getBoundingClientRect();
      console.log(this.containerRect);
    },
    startDrag(index) {
      this.dragging = true;
      this.x = this.y = 0;
      this.moving = index;
    },
    stopDrag() {
      this.dragging = false;
      this.x = this.y = 'no';
    },
    doDrag(moveCorner, event) {
      if (this.dragging) {

        // set moveCorner variable for simple mode
        moveCorner = moveCorner || this.moving;

        // Get container coords
        var left = this.containerRect.left;
        var top = this.containerRect.top;

        // get container height and weight
        var height = this.containerRect.height;
        var width = this.containerRect.width;

        // Get current corner and the direction of the corner being moved (vertical or horizontal)
        var element = this.corners[moveCorner];
        var direction = element.direction;

        // Get mouse position
        var x = event.clientX;
        var y = event.clientY;

        // Get how much the point should be moved 
        var move = (direction == "horizontal") ? x - left : y - top;
        move = move * 100;

        // Get move in percentage
        var percentage = (direction == "horizontal") ? move / width : move / height;

        if (percentage < 0)
          percentage = 0;

        if (percentage > 100)
          percentage = 100;

        percentage = Math.round(percentage);

        // For certain corners we need the calculate the position of the corner from the other side of the container. This is to displayed later in the CSS and will be applied to the border demo
        if (moveCorner == 1 || moveCorner == 2 || moveCorner == 6 || moveCorner == 7)
          element.amount = 100 - percentage + "%";
        else
          element.amount = percentage + "%";

        element.move = percentage + "%";

        if (this.simple == true) {
          switch (moveCorner) {
            case 0:
              this.doDrag(1, event);
              break;

            case 3:
              this.doDrag(2, event);
              break;

            case 4:
              this.doDrag(7, event);
              break;

            case 5:
              this.doDrag(6, event);
              break;
          }
        }
      }
    },
    getRadius: function() {
      var final = [];
      this.corners.forEach((corner, index) => {
        final.push(corner.amount);

        if (index == 3)
          final.push("/")
      });
      this.radius["border-radius"] = final.join(" ");
    },
    updateCorners: function() {
      this.corners = (this.simple) ? this.cornersSimple : this.cornersFull;
    }
  },
  watch: {
    corners: {
      handler: function() {
        this.getRadius();
      },
      deep: true
    }
  },
  mounted() {
    window.addEventListener('mouseup', this.stopDrag);
    window.addEventListener('resize', this.handleResize);
    var element = this.$refs.container;
    this.containerRect = element.getBoundingClientRect();

    this.updateCorners();
    this.getRadius();
  }
});
!
999px

Console