diff --git a/core/components/component.js b/core/components/component.js
index 2c36fbba9..93aa46ae3 100644
--- a/core/components/component.js
+++ b/core/components/component.js
@@ -47,7 +47,7 @@ Blockly.Component = function() {
    * been set by calling {@link #setRightToLeft} explicitly.
    * @private {?boolean}
    */
-  this.rightToLeft_ = Blockly.Component.defaultRightToLeft_;
+  this.rightToLeft_ = Blockly.Component.defaultRightToLeft;
 
   /**
    * Unique ID of the component, lazily initialized in {@link
@@ -77,42 +77,37 @@ Blockly.Component = function() {
   this.parent_ = null;
 
   /**
-   * Array of child components.  Lazily initialized on first use.  Must be kept
-   * in sync with `childIndex_`.  This property is strictly private and
-   * must not be accessed directly outside of this class!
+   * Array of child components.
+   * Must be kept in sync with `childIndex_`.  This property is strictly
+   * private and must not be accessed directly outside of this class!
    * @private {?Array.<?Blockly.Component>}
    */
-  this.children_ = null;
+  this.children_ = [];
 
   /**
    * Map of child component IDs to child components.  Used for constant-time
-   * random access to child components by ID.  Lazily initialized on first use.
+   * random access to child components by ID.
    * Must be kept in sync with `children_`.  This property is strictly
    * private and must not be accessed directly outside of this class!
    *
    * @private {?Object}
    */
-  this.childIndex_ = null;
+  this.childIndex_ = {};
 };
 
 
 /**
  * The default right to left value.
- * @type {?boolean}
- * @private
+ * @type {boolean}
+ * @package
  */
-Blockly.Component.defaultRightToLeft_ = false;
+Blockly.Component.defaultRightToLeft = false;
 
 /**
  * Errors thrown by the component.
  * @enum {string}
  */
 Blockly.Component.Error = {
-  /**
-   * Error when a method is not supported.
-   */
-  NOT_SUPPORTED: 'Method not supported',
-
   /**
    * Error when the component is already rendered and another render attempt is
    * made.
@@ -129,32 +124,7 @@ Blockly.Component.Error = {
    * Error when an attempt is made to add a child component at an out-of-bounds
    * index.  We don't support sparse child arrays.
    */
-  CHILD_INDEX_OUT_OF_BOUNDS: 'Child component index out of bounds',
-
-  /**
-   * Error when an attempt is made to remove a child component from a component
-   * other than its parent.
-   */
-  NOT_OUR_CHILD: 'Child is not in parent component',
-
-  /**
-   * Error when an operation requiring DOM interaction is made when the
-   * component is not in the document
-   */
-  NOT_IN_DOCUMENT: 'Operation not supported while component is not in document'
-};
-
-/**
- * Set the default right-to-left value. This causes all component's created from
- * this point forward to have the given value. This is useful for cases where
- * a given page is always in one directionality, avoiding unnecessary
- * right to left determinations.
- * @param {?boolean} rightToLeft Whether the components should be rendered
- *     right-to-left. Null iff components should determine their directionality.
- * @package
- */
-Blockly.Component.setDefaultRightToLeft = function(rightToLeft) {
-  Blockly.Component.defaultRightToLeft_ = rightToLeft;
+  CHILD_INDEX_OUT_OF_BOUNDS: 'Child component index out of bounds'
 };
 
 /**
@@ -196,11 +166,7 @@ Blockly.Component.prototype.setElementInternal = function(element) {
 /**
  * Sets the parent of this component to use for event bubbling.  Throws an error
  * if the component already has a parent or if an attempt is made to add a
- * component to itself as a child.  Callers must use `removeChild`
- * or `removeChildAt` to remove components from their containers before
- * calling this method.
- * @see Blockly.Component#removeChild
- * @see Blockly.Component#removeChildAt
+ * component to itself as a child.
  * @param {Blockly.Component} parent The parent component.
  * @protected
  */
@@ -212,8 +178,7 @@ Blockly.Component.prototype.setParent = function(parent) {
 
   if (parent && this.parent_ && this.id_ && this.parent_.getChild(this.id_) &&
       this.parent_ != parent) {
-    // This component is already the child of some parent, so it should be
-    // removed using removeChild/removeChildAt first.
+    // This component is already the child of some parent.
     throw Error(Blockly.Component.Error.PARENT_UNABLE_TO_BE_SET);
   }
 
@@ -479,12 +444,6 @@ Blockly.Component.prototype.addChildAt = function(child, index, opt_render) {
     throw Error(Blockly.Component.Error.CHILD_INDEX_OUT_OF_BOUNDS);
   }
 
-  // Create the index and the child array on first use.
-  if (!this.childIndex_ || !this.children_) {
-    this.childIndex_ = {};
-    this.children_ = [];
-  }
-
   // Moving child within component, remove old reference.
   this.childIndex_[child.getId()] = child;
   if (child.getParent() == this) {
@@ -554,16 +513,12 @@ Blockly.Component.prototype.getContentElement = function() {
  * @protected
  */
 Blockly.Component.prototype.isRightToLeft = function() {
-  if (this.rightToLeft_ == null) {
-    this.rightToLeft_ = Blockly.utils.style.isRightToLeft(
-        this.inDocument_ ? this.element_ : document.body);
-  }
   return this.rightToLeft_;
 };
 
 /**
  * Set is right-to-left. This function should be used if the component needs
- * to know the rendering direction during dom creation (i.e. before
+ * to know the rendering direction during DOM creation (i.e. before
  * {@link #enterDocument} is called and is right-to-left is set).
  * @param {boolean} rightToLeft Whether the component is rendered
  *     right-to-left.
@@ -582,7 +537,7 @@ Blockly.Component.prototype.setRightToLeft = function(rightToLeft) {
  * @protected
  */
 Blockly.Component.prototype.hasChildren = function() {
-  return !!this.children_ && this.children_.length != 0;
+  return this.children_.length != 0;
 };
 
 /**
@@ -591,7 +546,7 @@ Blockly.Component.prototype.hasChildren = function() {
  * @protected
  */
 Blockly.Component.prototype.getChildCount = function() {
-  return this.children_ ? this.children_.length : 0;
+  return this.children_.length;
 };
 
 /**
@@ -602,10 +557,8 @@ Blockly.Component.prototype.getChildCount = function() {
  */
 Blockly.Component.prototype.getChild = function(id) {
   // Use childIndex_ for O(1) access by ID.
-  return (this.childIndex_ && id) ?
-      /** @type {Blockly.Component} */ (
-        this.childIndex_[id]) ||
-        null : null;
+  return id ?
+      /** @type {Blockly.Component} */ (this.childIndex_[id]) || null : null;
 };
 
 /**
@@ -616,7 +569,7 @@ Blockly.Component.prototype.getChild = function(id) {
  */
 Blockly.Component.prototype.getChildAt = function(index) {
   // Use children_ for access by index.
-  return this.children_ ? this.children_[index] || null : null;
+  return this.children_[index] || null;
 };
 
 /**
@@ -631,10 +584,8 @@ Blockly.Component.prototype.getChildAt = function(index) {
  * @protected
  */
 Blockly.Component.prototype.forEachChild = function(f, opt_obj) {
-  if (this.children_) {
-    for (var i = 0; i < this.children_.length; i++) {
-      f.call(/** @type {?} */ (opt_obj), this.children_[i], i);
-    }
+  for (var i = 0; i < this.children_.length; i++) {
+    f.call(/** @type {?} */ (opt_obj), this.children_[i], i);
   }
 };
 
@@ -646,96 +597,5 @@ Blockly.Component.prototype.forEachChild = function(f, opt_obj) {
  * @protected
  */
 Blockly.Component.prototype.indexOfChild = function(child) {
-  return (this.children_ && child) ? this.children_.indexOf(child) : -1;
-};
-
-/**
- * Removes the given child from this component, and returns it.  Throws an error
- * if the argument is invalid or if the specified child isn't found in the
- * parent component.  The argument can either be a string (interpreted as the
- * ID of the child component to remove) or the child component itself.
- *
- * If `opt_unrender` is true, calls {@link Blockly.Component#exitDocument}
- * on the removed child, and subsequently detaches the child's DOM from the
- * document.  Otherwise it is the caller's responsibility to clean up the child
- * component's DOM.
- *
- * @see Blockly.Component#removeChildAt
- * @param {string|Blockly.Component|null} child The ID of the child to remove,
- *    or the child component itself.
- * @param {boolean=} opt_unrender If true, calls `exitDocument` on the
- *    removed child component, and detaches its DOM from the document.
- * @return {Blockly.Component} The removed component, if any.
- * @protected
- */
-Blockly.Component.prototype.removeChild = function(child, opt_unrender) {
-  if (child) {
-    // Normalize child to be the object and id to be the ID string.  This also
-    // ensures that the child is really ours.
-    var id = (typeof child === 'string' || child instanceof String) ?
-        String(child) : child.getId();
-    child = this.getChild(id);
-
-    if (id && child) {
-      delete this.childIndex_[id];
-      var index = this.children_.indexOf(child);
-      if (index > -1) {
-        this.children_.splice(index, 1);
-      }
-
-      if (opt_unrender) {
-        // Remove the child component's DOM from the document.  We have to call
-        // exitDocument first (see documentation).
-        child.exitDocument();
-        if (child.element_) {
-          Blockly.utils.dom.removeNode(child.element_);
-        }
-      }
-
-      // Child's parent must be set to null after exitDocument is called
-      // so that the child can unlisten to its parent if required.
-      child.setParent(null);
-    }
-  }
-
-  if (!child) {
-    throw Error(Blockly.Component.Error.NOT_OUR_CHILD);
-  }
-
-  return /** @type {!Blockly.Component} */ (child);
-};
-
-/**
- * Removes the child at the given index from this component, and returns it.
- * Throws an error if the argument is out of bounds, or if the specified child
- * isn't found in the parent.  See {@link Blockly.Component#removeChild} for
- * detailed semantics.
- *
- * @see Blockly.Component#removeChild
- * @param {number} index 0-based index of the child to remove.
- * @param {boolean=} opt_unrender If true, calls `exitDocument` on the
- *    removed child component, and detaches its DOM from the document.
- * @return {Blockly.Component} The removed component, if any.
- * @protected
- */
-Blockly.Component.prototype.removeChildAt = function(index, opt_unrender) {
-  // removeChild(null) will throw error.
-  return this.removeChild(this.getChildAt(index), opt_unrender);
-};
-
-/**
- * Removes every child component attached to this one and returns them.
- *
- * @see Blockly.Component#removeChild
- * @param {boolean=} opt_unrender If true, calls {@link #exitDocument} on the
- *    removed child components, and detaches their DOM from the document.
- * @return {!Array.<Blockly.Component>} The removed components if any.
- * @protected
- */
-Blockly.Component.prototype.removeChildren = function(opt_unrender) {
-  var removedChildren = [];
-  while (this.hasChildren()) {
-    removedChildren.push(this.removeChildAt(0, opt_unrender));
-  }
-  return removedChildren;
+  return this.children_.indexOf(child);
 };
diff --git a/core/components/menu/menuitem.js b/core/components/menu/menuitem.js
index acb71d9fb..3ca493bcf 100644
--- a/core/components/menu/menuitem.js
+++ b/core/components/menu/menuitem.js
@@ -82,12 +82,11 @@ Blockly.MenuItem.prototype.createDom = function() {
       (!this.enabled_ ? 'goog-menuitem-disabled ' : '') +
       (this.checked_ ? 'goog-option-selected ' : '') +
       (this.isRightToLeft() ? 'goog-menuitem-rtl ' : ''));
-  element.setAttribute('style', 'user-select: none');
 
   var content = this.getContentWrapperDom();
   element.appendChild(content);
 
-  // Add a cheeckbox for checkable menu items.
+  // Add a checkbox for checkable menu items.
   var checkboxDom = this.getCheckboxDom();
   if (checkboxDom) {
     content.appendChild(checkboxDom);
@@ -113,7 +112,6 @@ Blockly.MenuItem.prototype.getCheckboxDom = function() {
   }
   var menuItemCheckbox = document.createElement('div');
   menuItemCheckbox.setAttribute('class', 'goog-menuitem-checkbox');
-  menuItemCheckbox.setAttribute('style', 'user-select: none;');
   return menuItemCheckbox;
 };
 
@@ -136,7 +134,6 @@ Blockly.MenuItem.prototype.getContentDom = function() {
 Blockly.MenuItem.prototype.getContentWrapperDom = function() {
   var contentWrapper = document.createElement('div');
   contentWrapper.setAttribute('class', 'goog-menuitem-content');
-  contentWrapper.setAttribute('style', 'user-select: none;');
   return contentWrapper;
 };
 
@@ -212,15 +209,6 @@ Blockly.MenuItem.prototype.setChecked = function(checked) {
   }
 };
 
-/**
- * Returns true if the component is currently highlighted, false otherwise.
- * @return {boolean} Whether the component is highlighted.
- * @package
- */
-Blockly.MenuItem.prototype.isHighlighted = function() {
-  return this.highlight_;
-};
-
 /**
  * Highlights or unhighlights the component.
  * @param {boolean} highlight Whether to highlight or unhighlight the component.
diff --git a/core/components/tree/basenode.js b/core/components/tree/basenode.js
index 456cffde2..e61e932c2 100644
--- a/core/components/tree/basenode.js
+++ b/core/components/tree/basenode.js
@@ -233,8 +233,7 @@ Blockly.tree.BaseNode.prototype.exitDocument = function() {
  * The method assumes that the child doesn't have parent node yet.
  * @override
  */
-Blockly.tree.BaseNode.prototype.addChildAt = function(
-    child, index) {
+Blockly.tree.BaseNode.prototype.addChildAt = function(child, index) {
   child = /** @type {Blockly.tree.BaseNode} */ (child);
   var prevNode = this.getChildAt(index - 1);
   var nextNode = this.getChildAt(index);
@@ -293,21 +292,15 @@ Blockly.tree.BaseNode.prototype.addChildAt = function(
 };
 
 /**
- * Adds a node as a child to the current node.
+ * Appends a node as a child to the current node.
  * @param {Blockly.tree.BaseNode} child The child to add.
- * @param {Blockly.tree.BaseNode=} opt_before If specified, the new child is
- *    added as a child before this one. If not specified, it's appended to the
- *    end.
- * @return {!Blockly.tree.BaseNode} The added child.
  * @package
  */
-Blockly.tree.BaseNode.prototype.add = function(child, opt_before) {
+Blockly.tree.BaseNode.prototype.add = function(child) {
   if (child.getParent()) {
-    child.getParent().removeChild(child);
+    throw Error(Blockly.Component.Error.PARENT_UNABLE_TO_BE_SET);
   }
-  this.addChildAt(
-      child, opt_before ? this.indexOfChild(opt_before) : this.getChildCount());
-  return child;
+  this.addChildAt(child, this.getChildCount());
 };
 
 /**
diff --git a/core/css.js b/core/css.js
index 9ec99dfad..28dc29f9d 100644
--- a/core/css.js
+++ b/core/css.js
@@ -136,7 +136,6 @@ Blockly.Css.CONTENT = [
 
   '.blocklyNonSelectable {',
     'user-select: none;',
-    '-moz-user-select: none;',
     '-ms-user-select: none;',
     '-webkit-user-select: none;',
   '}',
@@ -431,7 +430,6 @@ Blockly.Css.CONTENT = [
   */
   '.blocklySvg text, .blocklyBlockDragSurface text {',
     'user-select: none;',
-    '-moz-user-select: none;',
     '-ms-user-select: none;',
     '-webkit-user-select: none;',
     'cursor: inherit;',
@@ -699,7 +697,6 @@ Blockly.Css.CONTENT = [
     'overflow-y: auto;',
     'position: absolute;',
     'user-select: none;',
-    '-moz-user-select: none;',
     '-ms-user-select: none;',
     '-webkit-user-select: none;',
     'z-index: 70;', /* so blocks go under toolbox when dragging */
diff --git a/core/field_colour.js b/core/field_colour.js
index 7dc9f4a23..62057cc95 100644
--- a/core/field_colour.js
+++ b/core/field_colour.js
@@ -504,7 +504,7 @@ Blockly.FieldColour.prototype.setHighlightedCell_ = function(cell, index) {
   if (highlighted) {
     Blockly.utils.dom.removeClass(highlighted, 'blocklyColourHighlighted');
   }
-  // Highight new item.
+  // Highlight new item.
   Blockly.utils.dom.addClass(cell, 'blocklyColourHighlighted');
   // Set new highlighted index.
   this.highlightedIndex_ = index;
diff --git a/core/flyout_base.js b/core/flyout_base.js
index 04d104969..2ab6ecb64 100644
--- a/core/flyout_base.js
+++ b/core/flyout_base.js
@@ -371,7 +371,7 @@ Blockly.Flyout.prototype.updateDisplay_ = function() {
     show = this.isVisible();
   }
   this.svgGroup_.style.display = show ? 'block' : 'none';
-  // Update the scrollbar's visiblity too since it should mimic the
+  // Update the scrollbar's visibility too since it should mimic the
   // flyout's visibility.
   this.scrollbar_.setContainerVisible(show);
 };
diff --git a/core/inject.js b/core/inject.js
index 5885201bf..a7d49b95f 100644
--- a/core/inject.js
+++ b/core/inject.js
@@ -96,7 +96,7 @@ Blockly.createDom_ = function(container, options) {
   // then manually positions content in RTL as needed.
   container.setAttribute('dir', 'LTR');
   // Set the default direction for Components to use.
-  Blockly.Component.setDefaultRightToLeft(options.RTL);
+  Blockly.Component.defaultRightToLeft = options.RTL;
 
   // Load CSS.
   Blockly.Css.inject(options.hasCss, options.pathToMedia);
diff --git a/core/toolbox.js b/core/toolbox.js
index 5a01affec..b32614521 100644
--- a/core/toolbox.js
+++ b/core/toolbox.js
@@ -198,6 +198,19 @@ Blockly.Toolbox.prototype.init = function() {
   this.config_['cleardotPath'] = workspace.options.pathToMedia + '1x1.gif';
   this.config_['cssCollapsedFolderIcon'] =
       'blocklyTreeIconClosed' + (workspace.RTL ? 'Rtl' : 'Ltr');
+  this.renderTree(workspace.options.languageTree);
+};
+
+/**
+ * Fill the toolbox with categories and blocks.
+ * @param {!Node} languageTree DOM tree of blocks.
+ * @package
+ */
+Blockly.Toolbox.prototype.renderTree = function(languageTree) {
+  if (this.tree_) {
+    this.tree_.dispose();  // Delete any existing content.
+    this.lastCategory_ = null;
+  }
   var tree = new Blockly.tree.TreeControl(this,
       /** @type {!Blockly.tree.BaseNode.Config} */ (this.config_));
   this.tree_ = tree;
@@ -205,8 +218,18 @@ Blockly.Toolbox.prototype.init = function() {
   tree.onBeforeSelected(this.handleBeforeTreeSelected_);
   tree.onAfterSelected(this.handleAfterTreeSelected_);
   var openNode = null;
-  if (workspace.options.languageTree) {
-    openNode = this.populate_(workspace.options.languageTree);
+  if (languageTree) {
+    this.tree_.blocks = [];
+    this.hasColours_ = false;
+    var openNode =
+      this.syncTrees_(languageTree, this.tree_, this.workspace_.options.pathToMedia);
+
+    if (this.tree_.blocks.length) {
+      throw Error('Toolbox cannot have both blocks and categories ' +
+          'in the root level.');
+    }
+    // Fire a resize event since the toolbox may have changed width and height.
+    this.workspace_.resizeContents();
   }
   tree.render(this.HtmlDiv);
   if (openNode) {
@@ -377,29 +400,6 @@ Blockly.Toolbox.prototype.position = function() {
   this.flyout_.position();
 };
 
-/**
- * Fill the toolbox with categories and blocks.
- * @param {!Node} newTree DOM tree of blocks.
- * @return {Blockly.tree.BaseNode} Tree node to open at startup (or null).
- * @private
- */
-Blockly.Toolbox.prototype.populate_ = function(newTree) {
-  this.tree_.removeChildren();  // Delete any existing content.
-  this.tree_.blocks = [];
-  this.hasColours_ = false;
-  var openNode =
-    this.syncTrees_(newTree, this.tree_, this.workspace_.options.pathToMedia);
-
-  if (this.tree_.blocks.length) {
-    throw Error('Toolbox cannot have both blocks and categories ' +
-        'in the root level.');
-  }
-
-  // Fire a resize event since the toolbox may have changed width and height.
-  this.workspace_.resizeContents();
-  return openNode;
-};
-
 /**
  * Sync trees of the toolbox.
  * @param {!Node} treeIn DOM tree of blocks.
diff --git a/core/workspace_svg.js b/core/workspace_svg.js
index cb044addb..1d291e382 100644
--- a/core/workspace_svg.js
+++ b/core/workspace_svg.js
@@ -1727,10 +1727,7 @@ Blockly.WorkspaceSvg.prototype.updateToolbox = function(tree) {
       throw Error('Existing toolbox has no categories.  Can\'t change mode.');
     }
     this.options.languageTree = tree;
-    var openNode = this.toolbox_.populate_(tree);
-    this.toolbox_.addColour_();
-    this.toolbox_.position();
-    this.toolbox_.tree_.setSelectedItem(openNode);
+    this.toolbox_.renderTree(tree);
   } else {
     if (!this.flyout_) {
       throw Error('Existing toolbox has categories.  Can\'t change mode.');
@@ -1750,7 +1747,7 @@ Blockly.WorkspaceSvg.prototype.markFocused = function() {
     Blockly.mainWorkspace = this;
     // We call e.preventDefault in many event handlers which means we
     // need to explicitly grab focus (e.g from a textarea) because
-    // the browser will not do it for us.  How to do this is browser dependant.
+    // the browser will not do it for us.  How to do this is browser dependent.
     this.setBrowserFocus();
   }
 };
@@ -1760,7 +1757,7 @@ Blockly.WorkspaceSvg.prototype.markFocused = function() {
  * @private
  */
 Blockly.WorkspaceSvg.prototype.setBrowserFocus = function() {
-  // Blur whatever was focused since explcitly grabbing focus below does not
+  // Blur whatever was focused since explicitly grabbing focus below does not
   // work in Edge.
   if (document.activeElement) {
     document.activeElement.blur();