anchor.diff:                D.DIV({
                   // Make our first image set visible
                   // DEV: If class names get too complex, use `classnames` library
  -                class: imgSet.imagesEqual ? 'collapse well' : 'collapse in well',
  -                'data-image-set-expand': true
  +                class: imgSet.imagesEqual ? 'image-set__collapse collapse well' : 'image-set__collapse collapse in well'
                 }, [
                   // Action buttons
                   D.DIV([
  
positive.code:               var currentImg = imageSetInfo.currentImg;
                var refImg = imageSetInfo.refImg;
  
                // Create a canvas
                // https://github.com/scijs/get-pixels/blob/7c447cd979637b31e47e148f238a1e71611af481/dom-pixels.js#L14-L18
                var canvasEl = D.CANVAS({class: 'updated-ref'});
                canvasEl.width = refImg.naturalWidth;
                canvasEl.height = refImg.naturalHeight;
                var context = canvasEl.getContext('2d');
  
                // Load our reference into the canvas
                // https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/drawImage
                context.drawImage(refImg, 0, 0);
  
                // Update our selected portion on reference and draw image in clipping
                // DEV: This is probably the most efficient way (outside of web workers) because
                //   we would have to draw image twice no matter what
                //   Maybe there's double pixel updates but I don't thinks so
                // DEV: Performance alternatives we thought of but aren't needed
                //   Extract image from 2nd canvas via `ndarray`
                //   Use web workers
                //   Requesting server do it via `get-pixels` and `save-pixels`
                // DEV: Slowest part is drawing initial image above
                // https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/rect
                context.rect(targetArea.left, targetArea.top, targetArea.width, targetArea.height);
                context.clip();
                context.drawImage(currentImg, 0, 0);
  
                // Duplicate reference image and shrink both canvas/image for output
                canvasEl.style.width = '100%';
  
negative.code:           var btnEl = evt.target;
            var $expectedImgSet = $(btnEl).closest('[data-image-set]');
            var expectedImgSetEl = $expectedImgSet[0];
            var expectedImgSetCollapseEl = $expectedImgSet.find('.image-set__collapse')[0];
  
            // Find our target area
            var imgOverlay = expectedImgSetEl.imgOverlay;
  