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HTML

              
                .masonry-root
  - for i in (1..512)
    .masonry-cell
      - for j in (1..1)
        .masonry-item #{i}

              
            
!

CSS

              
                body {
  margin: 8px;
}

.masonry-root {
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
  .masonry-cell {
    flex: 1;
    margin: 4px;
    .masonry-item {
      background: #EC985A;
      color: white;
      margin: 4px;
      text-align: center;
      font-family: system-ui;
      font-weight: 900;
      font-size: 32px;
    }
  } 
  @for $i from 1 through 512 { 
    .masonry-cell:nth-child(#{$i}) {
      .masonry-item {
        $h: (random(128) + 64) + px;
        height: $h;
        line-height: $h;
      }
    }
  }
}

              
            
!

JS

              
                
var minColWidth = 128;
var roots;

function onLoad() {
  var rootElements = document.getElementsByClassName('masonry-root');
  roots = Array.prototype.map.call(rootElements, function(rootElement) {
    var cellElements = rootElement.getElementsByClassName('masonry-cell');
    var cells = Array.prototype.map.call(cellElements, function(cellElement) {
      var style = getComputedStyle(cellElement);
      return {
        'element': cellElement,
        'outerHeight': parseInt(style.marginTop) + cellElement.offsetHeight + parseInt(style.marginBottom)
      };
    });
    return {
      'element': rootElement,
      'noOfColumns': 0,
      'cells': cells
    };
  });
  
  // do the first layout
  onResize();
}

function onResize() {
  for (let root of roots) {
    
    // only layout when the number of columns has changed
    var newNoOfColumns = Math.floor(root.element.offsetWidth / minColWidth);
    if (newNoOfColumns != root.noOfColumns) {

      // initialize
      root.noOfColumns = newNoOfColumns;
      var columns = Array.from(new Array(root.noOfColumns)).map( function(column) {
        return {
          'cells': new Array(),
          'outerHeight': 0
        };
      });
      
      // divide...
      for (let cell of root.cells) {
        var minOuterHeight = Math.min(...columns.map( function(column) {
          return column.outerHeight;
        }));
        var column = columns.find( function(column) {
          return column.outerHeight == minOuterHeight;
        });
        column.cells.push(cell);
        column.outerHeight += cell.outerHeight;
      }
      
      // calculate masonry height
      var masonryHeight = Math.max(...columns.map( function(column) {
        return column.outerHeight;
      }));
      
      // ...and conquer
      var order = 0;
      for (let column of columns) {
        for (let cell of column.cells) {
          cell.element.style.order = order++;
          // set the cell's flex-basis to 0
          cell.element.style.flexBasis = 0;
        }
        // set flex-basis of the last cell to fill the
        // leftover space at the bottom of the column
        // to prevent the first cell of the next column
        // to be rendered at the bottom of this column
        column.cells[column.cells.length - 1].element.style.flexBasis = column.cells[column.cells.length - 1].element.offsetHeight + masonryHeight - column.outerHeight - 1 + 'px';
      }

      // set the masonry height to trigger
      // re-rendering of all cells over columns
      // one pixel more than the tallest column
      root.element.style.maxHeight = masonryHeight + 1 + 'px';

      console.log(columns.map( function(column) {
        return column.outerHeight;
      }));
      console.log(root.element.style.maxHeight);
    }
  }
}

// subscribe to load and resize events
window.addEventListener('load', onLoad);
window.addEventListener('resize', onResize);

              
            
!
999px

Console