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Click-to-zoom with CSS transform, and pure-JS pan functionality.

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.2.2/jquery.min.js"></script>

<button id="zoom-out">Zoom out</button>

<div id="container">
  <div id="inner-container">
    <div id="zoom-container">
      <div class="box red">A</div>
      <div class="box blue">B</div>
      <div class="box green">C</div>
      <div class="box black">D</div>
    </div>
  </div>
</div>

CSS

* {
  margin: 0;
}

body,
html {
  height: 100%;
}

#container {
  height: 100%;
  width: 50%;
  margin: 0 auto;
}

#inner-container {
  width: 100%;
  height: 100%;
}

#zoom-container {
  height: 100%;
  width: 100%;
  transition: transform 0.2s ease-in-out;
}

.box {
  float: left;
  width: 50%;
  height: 50%;
  color: white;
  text-align: center;
  display: block;
}

.red {
  background: red;
}

.blue {
  background: blue;
}

.green {
  background: green;
}

.black {
  background: black;
}

JavaScript

//
// credit for the approach goes to
//
//   https://stackoverflow.com/questions/35252249/move-drag-pan-and-zoom-object-image-or-div-in-pure-js#comment58224460_35253567
//
// and the corresponding example:
//
//  https://jsfiddle.net/j8kLz6wm/1/
//

// in a real-world setting, you
// wouldn't keep this information
// on window. this is just for
// the demonstration.
window.zoomedIn = false;

// stores the initial translate values after clicking on a box
window.translateY = null;
window.translateX = null;

// stores the incremental translate values based on
// applying the initial translate values + delta
window.lastTranslateY = null;
window.lastTranslateX = null;

// cursor position relative to the container, at
// the time the drag started
window.dragStartX = null;
window.dragStartY = null;

var handleDragStart = function(element, xCursor, yCursor) {
  window.dragStartX = xCursor - element.offsetLeft;
  window.dragStartY = yCursor - element.offsetTop;

  // disable transition animations, since we're starting a drag
  $("#zoom-container").css("transition", "none");
};

var handleDragEnd = function() {
  window.dragStartX = null;
  window.dragStartY = null;
  // remove the individual element's styling for transitions
  // which brings back the stylesheet's default of animating.
  $("#zoom-container").css("transition", "");

  // keep track of the translate values we arrived at
  window.translateY = window.lastTranslateY;
  window.translateX = window.lastTranslateX;
};

var handleDragMove = function(xCursor, yCursor) {
  var deltaX = xCursor - window.dragStartX;
  var deltaY = yCursor - window.dragStartY;
  
  var translateY = window.translateY + (deltaY / 2);
  // the subtracted value here is to keep the letter in the center
  var translateX = window.translateX + (deltaX / 2) - (0.25 * $("#inner-container")[0].clientWidth);
  
  // fudge factor, probably because of percentage
  // width/height problems. couldn't really trace down
  // the underlying cause....