message: add comments to fields
diff: @@ -31,9 +31,15 @@ function clamp(height, min, max) {
  export class Layout {
  constructor(applyHeight, initialSizes, collapsedState) {
  + // callback to set height of section
  this._applyHeight = applyHeight;
  + // list of {id, count} objects,
  + // determines sections and order of them
  this._sections = [];
  + // stores collapsed by id
  this._collapsedState = Object.assign({}, collapsedState);
  + // total available height to the layout
  + // (including resize handles, ...)
  this._availableHeight = 0;
  // heights stored by section section id
  this._sectionHeights = Object.assign({}, initialSizes);
  