Addressing review comments.

packages/cells/src/collapser.tsx

@@ -117,8 +117,8 @@ class InputCollapser extends Collapser {
    * Construct a new input collapser.
    */
   constructor() {
-      super();
-      this.addClass(INPUT_COLLAPSER);
+    super();
+    this.addClass(INPUT_COLLAPSER);
   }
 
   /**
@@ -153,8 +153,8 @@ class InputCollapser extends Collapser {
 export
 class OutputCollapser extends Collapser {
     /**
-    * Construct a new output collapser.
-    */
+     * Construct a new output collapser.
+     */
     constructor() {
       super();
       this.addClass(OUTPUT_COLLAPSER);

packages/cells/src/headerfooter.ts

@@ -50,7 +50,7 @@ interface ICellFooter extends Widget {}
 
 
 /**
- * Default implementation of a cell header.
+ * Default implementation of a cell footer.
  */
 export
 class CellFooter extends Widget implements ICellFooter {

packages/cells/src/placeholder.tsx

@@ -14,7 +14,7 @@ import {
 
 
 /**
- * The CSS class added to placehodlers.
+ * The CSS class added to placeholders.
  */
 const PLACEHOLDER_CLASS = 'jp-Placeholder';
 

packages/cells/src/widget.ts

@@ -280,10 +280,10 @@ class Cell extends Widget {
     }
     let layout = this._inputWrapper.layout as PanelLayout;
     if (value) {
-      layout.removeWidget(this._input);
+      this._input.parent = null;
       layout.addWidget(this._inputPlaceholder);
     } else {
-      layout.removeWidget(this._inputPlaceholder);
+      this._inputPlaceholder.parent = null;
       layout.addWidget(this._input);
     }
     this._inputHidden = value;

packages/cells/style/headerfooter.css

@@ -9,7 +9,7 @@
 |----------------------------------------------------------------------------*/
 
 
-/* Hidden by zero height be default */
+/* Hidden by zero height by default */
 .jp-CellHeader, .jp-CellFooter  {
   height: 0px;
   width: 100%;