diff --git a/accessible/media/accessible.css b/accessible/media/accessible.css
index 77918fa90..d068253bf 100644
--- a/accessible/media/accessible.css
+++ b/accessible/media/accessible.css
@@ -49,7 +49,7 @@
 .blocklyModal {
   background-color: #fefefe;
   border: 1px solid #888;
-  margin: 15% auto;
+  margin: 10% auto;
   max-width: 600px;
   padding: 20px;
   width: 60%;
diff --git a/accessible/sidebar.component.js b/accessible/sidebar.component.js
index 9d70afbbe..70fc806e5 100644
--- a/accessible/sidebar.component.js
+++ b/accessible/sidebar.component.js
@@ -37,11 +37,18 @@ blocklyApp.SidebarComponent = ng.core.Component({
           {{buttonConfig.text}}
         </button>
       </span>
-      <button (click)="showToolboxModal()"
+      <button (click)="showToolboxModalForAttachToMarkedSpot()"
+              [attr.disabled]="isAnyConnectionMarked() ? undefined : 'disabled'"
+              [attr.aria-disabled]="!isAnyConnectionMarked()"
+              class="blocklySidebarButton">
+        Attach new block to link...
+      </button>
+      <button (click)="showToolboxModalForCreateNewGroup()"
               class="blocklySidebarButton">
         Create new block group...
       </button>
       <button id="clear-workspace" (click)="workspace.clear()"
+              [attr.disabled]="isWorkspaceEmpty() ? 'disabled' : undefined"
               [attr.aria-disabled]="isWorkspaceEmpty()"
               class="blocklySidebarButton">
         {{'CLEAR_WORKSPACE'|translate}}
@@ -55,9 +62,10 @@ blocklyApp.SidebarComponent = ng.core.Component({
   constructor: [
     blocklyApp.NotificationsService, blocklyApp.TreeService,
     blocklyApp.UtilsService, blocklyApp.ToolboxModalService,
+    blocklyApp.ClipboardService,
     function(
         _notificationsService, _treeService, _utilsService,
-        _toolboxModalService) {
+        _toolboxModalService, _clipboardService) {
       // ACCESSIBLE_GLOBALS is a global variable defined by the containing
       // page. It should contain a key, customSidebarButtons, describing
       // additional buttons that should be displayed after the default ones.
@@ -70,8 +78,12 @@ blocklyApp.SidebarComponent = ng.core.Component({
       this.treeService = _treeService;
       this.utilsService = _utilsService;
       this.toolboxModalService = _toolboxModalService;
+      this.clipboardService = _clipboardService;
     }
   ],
+  isAnyConnectionMarked: function() {
+    return this.clipboardService.isAnyConnectionMarked();
+  },
   handleButtonClick: function(buttonConfig) {
     buttonConfig.action();
     if (buttonConfig.onClickNotification) {
@@ -85,7 +97,64 @@ blocklyApp.SidebarComponent = ng.core.Component({
   isWorkspaceEmpty: function() {
     return this.utilsService.isWorkspaceEmpty();
   },
-  showToolboxModal: function() {
-    this.toolboxModalService.showModal();
+  showToolboxModalForAttachToMarkedSpot: function() {
+    var that = this;
+    this.toolboxModalService.showModal(function(opt_block) {
+      if (!opt_block) {
+        return;
+      }
+
+      var block = opt_block;
+      var blockDescription = that.utilsService.getBlockDescription(block);
+
+      // Clean up the active desc for the destination tree.
+      var oldDestinationTreeId = that.treeService.getTreeIdForBlock(
+          that.clipboardService.getMarkedConnectionBlock().id);
+      that.treeService.clearActiveDesc(oldDestinationTreeId);
+      var newBlockId = that.clipboardService.pasteToMarkedConnection(block);
+
+      // Invoke a digest cycle, so that the DOM settles.
+      setTimeout(function() {
+        that.treeService.focusOnBlock(newBlockId);
+
+        var newDestinationTreeId = that.treeService.getTreeIdForBlock(
+            newBlockId);
+        if (newDestinationTreeId != oldDestinationTreeId) {
+          // It is possible for the tree ID for the pasted block to change
+          // after the paste operation, e.g. when inserting a block between two
+          // existing blocks that are joined together. In this case, we need to
+          // also reset the active desc for the old destination tree.
+          that.treeService.initActiveDesc(oldDestinationTreeId);
+        }
+
+        that.notificationsService.setStatusMessage(
+            blockDescription + ' connected. ' +
+            'Now on copied block in workspace.');
+      });
+    }, function(block) {
+      return that.clipboardService.canBeCopiedToMarkedConnection(block);
+    });
+  },
+  showToolboxModalForCreateNewGroup: function() {
+    var that = this;
+    this.toolboxModalService.showModal(function(opt_block) {
+      if (!opt_block) {
+        return;
+      }
+
+      var block = opt_block;
+      var blockDescription = that.utilsService.getBlockDescription(block);
+      var xml = Blockly.Xml.blockToDom(block);
+      var newBlockId = Blockly.Xml.domToBlock(blocklyApp.workspace, xml).id;
+
+      setTimeout(function() {
+        that.treeService.focusOnBlock(newBlockId);
+        that.notificationsService.setStatusMessage(
+            blockDescription + ' added to workspace. ' +
+            'Now on added block in workspace.');
+      });
+    }, function(block) {
+      return true;
+    });
   }
 });
diff --git a/accessible/toolbox-modal.component.js b/accessible/toolbox-modal.component.js
index 426c903b9..9802c74ea 100644
--- a/accessible/toolbox-modal.component.js
+++ b/accessible/toolbox-modal.component.js
@@ -31,15 +31,17 @@ blocklyApp.ToolboxModalComponent = ng.core.Component({
         <!-- The $event.stopPropagation() here prevents the modal from
         closing when its interior is clicked. -->
         <div class="blocklyModal" (click)="$event.stopPropagation()" role="document">
-          <h3>Select a block for the new group...</h3>
+          <h3>Select a block...</h3>
 
           <div *ngFor="#toolboxCategory of toolboxCategories; #categoryIndex=index">
             <h4>{{toolboxCategory.categoryName}}</h4>
             <div class="blocklyModalButtonContainer"
                  *ngFor="#block of toolboxCategory.blocks; #blockIndex=index">
               <button [id]="getOptionId(getOverallIndex(categoryIndex, blockIndex))"
-                      (click)="selectBlock(categoryIndex, blockIndex); hideModal();"
-                      [ngClass]="{activeButton: activeButtonIndex == getOverallIndex(categoryIndex, blockIndex)}">
+                      (click)="hideModal(getBlock(categoryIndex, blockIndex))"
+                      [ngClass]="{activeButton: activeButtonIndex == getOverallIndex(categoryIndex, blockIndex)}"
+                      [attr.disabled]="isBlockAvailable(getBlock(categoryIndex, blockIndex)) ? undefined : 'disabled'"
+                      [attr.aria-disabled]="!isBlockAvailable(getBlock(categoryIndex, blockIndex))">
                 {{getBlockDescription(block)}}
               </button>
             </div>
@@ -77,12 +79,14 @@ blocklyApp.ToolboxModalComponent = ng.core.Component({
       this.firstBlockIndexes = [];
       this.activeButtonIndex = 0;
       this.totalNumBlocks = null;
+      this.isBlockAvailable = null;
 
       var that = this;
       this.toolboxModalService.registerPreShowHook(
-        function(toolboxCategories) {
+        function(toolboxCategories, isBlockAvailable) {
           that.modalIsVisible = true;
           that.toolboxCategories = toolboxCategories;
+          that.isBlockAvailable = isBlockAvailable;
 
           var cumulativeIndex = 0;
           that.toolboxCategories.forEach(function(category) {
@@ -105,14 +109,20 @@ blocklyApp.ToolboxModalComponent = ng.core.Component({
               var button = document.getElementById(
                   that.getOptionId(that.activeButtonIndex));
 
+              if (button.disabled) {
+                evt.preventDefault();
+                evt.stopPropagation();
+                return;
+              }
+
               for (var i = 0; i < that.toolboxCategories.length; i++) {
                 if (that.firstBlockIndexes[i + 1] > that.activeButtonIndex) {
                   var categoryIndex = i;
                   var blockIndex =
                       that.activeButtonIndex - that.firstBlockIndexes[i];
-
-                  that.selectBlock(categoryIndex, blockIndex);
-                  break;
+                  var block = that.getBlock(categoryIndex, blockIndex);
+                  that.hideModal(block);
+                  return;
                 }
               }
 
@@ -154,19 +164,8 @@ blocklyApp.ToolboxModalComponent = ng.core.Component({
   getOverallIndex: function(categoryIndex, blockIndex) {
     return this.firstBlockIndexes[categoryIndex] + blockIndex;
   },
-  selectBlock: function(categoryIndex, blockIndex) {
-    var block = this.toolboxCategories[categoryIndex].blocks[blockIndex];
-    var blockDescription = this.getBlockDescription(block);
-    var xml = Blockly.Xml.blockToDom(block);
-    var newBlockId = Blockly.Xml.domToBlock(blocklyApp.workspace, xml).id;
-
-    var that = this;
-    setTimeout(function() {
-      that.treeService.focusOnBlock(newBlockId);
-      that.notificationsService.setStatusMessage(
-          blockDescription + ' added to workspace. ' +
-          'Now on added block in workspace.');
-    });
+  getBlock: function(categoryIndex, blockIndex) {
+    return this.toolboxCategories[categoryIndex].blocks[blockIndex];
   },
   getBlockDescription: function(block) {
     return this.utilsService.getBlockDescription(block);
@@ -189,9 +188,9 @@ blocklyApp.ToolboxModalComponent = ng.core.Component({
     return 'toolbox-modal-option-' + this.totalNumBlocks;
   },
   // Closes the modal.
-  hideModal: function() {
+  hideModal: function(opt_block) {
     this.modalIsVisible = false;
     this.keyboardInputService.clearOverride();
-    this.toolboxModalService.hideModal();
+    this.toolboxModalService.hideModal(opt_block);
   }
 });
diff --git a/accessible/toolbox-modal.service.js b/accessible/toolbox-modal.service.js
index 525e6635d..ec52899f2 100644
--- a/accessible/toolbox-modal.service.js
+++ b/accessible/toolbox-modal.service.js
@@ -27,6 +27,7 @@ blocklyApp.ToolboxModalService = ng.core.Class({
   constructor: [function() {
     this.modalIsShown = false;
     this.onHideCallback = null;
+    this.isBlockAvailable = null;
     this.preShowHook = function() {
       throw Error(
           'A pre-show hook must be defined for the toolbox modal before it ' +
@@ -69,21 +70,20 @@ blocklyApp.ToolboxModalService = ng.core.Class({
   }],
   registerPreShowHook: function(preShowHook) {
     this.preShowHook = function() {
-      preShowHook(this.toolboxCategories);
+      preShowHook(this.toolboxCategories, this.isBlockAvailable);
     };
   },
   isModalShown: function() {
     return this.modalIsShown;
   },
-  showModal: function(onHideCallback) {
+  showModal: function(onHideCallback, isBlockAvailable) {
     this.onHideCallback = onHideCallback;
+    this.isBlockAvailable = isBlockAvailable;
     this.preShowHook();
     this.modalIsShown = true;
   },
-  hideModal: function() {
+  hideModal: function(opt_block) {
     this.modalIsShown = false;
-    if (this.onHideCallback) {
-      this.onHideCallback();
-    }
+    this.onHideCallback(opt_block);
   }
 });