From 0376350e266322b5c0779c3cc2842527ed81117e Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Tue, 11 Jul 2017 07:12:34 +0100
Subject: [PATCH 01/24] Allow none in accordion widget

---
 .../widgets/widget_selectioncontainer.py      |  2 +-
 .../controls/src/phosphor/currentselection.ts | 40 +++++++++++--------
 .../controls/src/widget_selectioncontainer.ts |  2 +-
 3 files changed, 25 insertions(+), 19 deletions(-)

diff --git a/ipywidgets/widgets/widget_selectioncontainer.py b/ipywidgets/widgets/widget_selectioncontainer.py
index e536cffe9d..404005ef30 100644
--- a/ipywidgets/widgets/widget_selectioncontainer.py
+++ b/ipywidgets/widgets/widget_selectioncontainer.py
@@ -16,7 +16,7 @@
 class _SelectionContainer(Box, CoreWidget):
     """Base class used to display multiple child widgets."""
     _titles = Dict(help="Titles of the pages").tag(sync=True)
-    selected_index = CInt(help="The index of the selected page.").tag(sync=True)
+    selected_index = CInt(help="The index of the selected page.", allow_none=True).tag(sync=True)
 
     # Public methods
     def set_title(self, index, title):
diff --git a/packages/controls/src/phosphor/currentselection.ts b/packages/controls/src/phosphor/currentselection.ts
index 0cf9af0301..ce9c24df60 100644
--- a/packages/controls/src/phosphor/currentselection.ts
+++ b/packages/controls/src/phosphor/currentselection.ts
@@ -93,7 +93,7 @@ class Selection<T> {
    */
   set value(value: T) {
     if (value === null) {
-      this.index = -1;
+      this.index = null;
     } else {
       this.index = ArrayExt.firstIndexOf(this._array, value);
     }
@@ -103,9 +103,9 @@ class Selection<T> {
    * Get the index of the currently selected item.
    *
    * #### Notes
-   * This will be `-1` if no item is selected.
+   * This will be `null` if no item is selected.
    */
-  get index(): number {
+  get index(): number | null {
     return this._index;
   }
 
@@ -115,14 +115,20 @@ class Selection<T> {
    * @param index - The index to select.
    *
    * #### Notes
-   * If the value is out of range, the index will be set to `-1`, which
+   * If the value is out of range, the index will be set to `null`, which
    * indicates no item is selected.
    */
-  set index(index: number) {
+  set index(index: number | null) {
     // Coerce the value to an index.
-    let i = Math.floor(index);
-    if (i < 0 || i >= this._array.length) {
-      i = -1;
+    let i;
+    if (index !== null) {
+        i = Math.floor(index);
+        if (i < 0 || i >= this._array.length) {
+            i = null;
+        }
+    }
+    else {
+      i = null;
     }
 
     // Bail early if the index will not change.
@@ -194,7 +200,7 @@ class Selection<T> {
 
     // Handle the behavior where the new item is always selected,
     // or the behavior where the new item is selected if needed.
-    if (bh === 'select-item' || (bh === 'select-item-if-needed' && ci === -1)) {
+    if (bh === 'select-item' || (bh === 'select-item-if-needed' && ci === null)) {
       this._index = i;
       this._value = item;
       this._previousValue = cv;
@@ -240,19 +246,19 @@ class Selection<T> {
     let pv = this._value;
 
     // Reset the current index and previous item.
-    this._index = -1;
+    this._index = null;
     this._value = null;
     this._previousValue = null;
 
     // If no item was selected, there's nothing else to do.
-    if (pi === -1) {
+    if (pi === null) {
       return;
     }
 
     // Emit the current changed signal.
     this._selectionChanged.emit({
       previousIndex: pi, previousValue: pv,
-      currentIndex: -1, currentValue: null
+      currentIndex: this._index, currentValue: this._value
     });
   }
 
@@ -283,12 +289,12 @@ class Selection<T> {
     // No item gets selected if the vector is empty.
     if (this._array.length === 0) {
       // Reset the current index and previous item.
-      this._index = -1;
+      this._index = null;
       this._value = null;
       this._previousValue = null;
       this._selectionChanged.emit({
         previousIndex: i, previousValue: item,
-        currentIndex: -1, currentValue: null
+        currentIndex: this._index, currentValue: this._value
       });
       return;
     }
@@ -334,12 +340,12 @@ class Selection<T> {
     }
 
     // Otherwise, no item gets selected.
-    this._index = -1;
+    this._index = null;
     this._value = null;
     this._previousValue = null;
     this._selectionChanged.emit({
       previousIndex: i, previousValue: item,
-      currentIndex: -1, currentValue: null
+      currentIndex: this._index, currentValue: this._value
     });
   }
 
@@ -348,7 +354,7 @@ class Selection<T> {
    */
   private _updateSelectedValue() {
     let i = this._index;
-    this._value = i !== -1 ? this._array[i] : null;
+    this._value = i !== null ? this._array[i] : null;
   }
 
   private _array: ReadonlyArray<T> = null;
diff --git a/packages/controls/src/widget_selectioncontainer.ts b/packages/controls/src/widget_selectioncontainer.ts
index 54e84ba6ca..55f26c5af6 100644
--- a/packages/controls/src/widget_selectioncontainer.ts
+++ b/packages/controls/src/widget_selectioncontainer.ts
@@ -156,7 +156,7 @@ class AccordionView extends DOMWidgetView {
         // tabs before updating so we don't get spurious changes in the index,
         // which would then set off another sync cycle.
         this.updatingChildren = true;
-        this.pWidget.selection.index = -1;
+        this.pWidget.selection.index = null;
         this.children_views.update(this.model.get('children'));
         this.update_selected_index();
         this.updatingChildren = false;

From 9b371aac890a84eb011123c12fdda69e5400bf45 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Tue, 11 Jul 2017 07:40:35 +0100
Subject: [PATCH 02/24] Allow none in tab panel

---
 packages/controls/src/phosphor/tabpanel.ts         | 4 ++--
 packages/controls/src/widget_selectioncontainer.ts | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/packages/controls/src/phosphor/tabpanel.ts b/packages/controls/src/phosphor/tabpanel.ts
index 209d03e462..0cdc3b8cfa 100644
--- a/packages/controls/src/phosphor/tabpanel.ts
+++ b/packages/controls/src/phosphor/tabpanel.ts
@@ -120,7 +120,7 @@ class TabPanel extends Widget {
    * Get the index of the currently selected tab.
    *
    * #### Notes
-   * This will be `-1` if no tab is selected.
+   * This will be `null` if no tab is selected.
    */
   get currentIndex(): number {
     return this.tabBar.currentIndex;
@@ -130,7 +130,7 @@ class TabPanel extends Widget {
    * Set the index of the currently selected tab.
    *
    * #### Notes
-   * If the index is out of range, it will be set to `-1`.
+   * If the index is out of range, it will be set to `null`.
    */
   set currentIndex(value: number) {
     this.tabBar.currentIndex = value;
diff --git a/packages/controls/src/widget_selectioncontainer.ts b/packages/controls/src/widget_selectioncontainer.ts
index 55f26c5af6..d8e26cec8f 100644
--- a/packages/controls/src/widget_selectioncontainer.ts
+++ b/packages/controls/src/widget_selectioncontainer.ts
@@ -339,7 +339,7 @@ class TabView extends DOMWidgetView {
         // tabs before updating so we don't get spurious changes in the index,
         // which would then set off another sync cycle.
         this.updatingTabs = true;
-        this.pWidget.currentIndex = -1;
+        this.pWidget.currentIndex = null;
         this.childrenViews.update(this.model.get('children'));
         this.pWidget.currentIndex = this.model.get('selected_index');
         this.updatingTabs = false;

From b6ec55626b39672dd9d3da8ed0fdc20d4bf5726a Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Tue, 11 Jul 2017 08:04:44 +0100
Subject: [PATCH 03/24] Add framework for current selection unit tests

---
 packages/controls/test/src/index.ts           |  1 +
 .../src/phosphor/currentselection_test.ts     | 23 +++++++++++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 packages/controls/test/src/phosphor/currentselection_test.ts

diff --git a/packages/controls/test/src/index.ts b/packages/controls/test/src/index.ts
index 8f17b16a72..dc06b6030e 100644
--- a/packages/controls/test/src/index.ts
+++ b/packages/controls/test/src/index.ts
@@ -2,3 +2,4 @@
 // Distributed under the terms of the Modified BSD License.
 
 import './widget_date_test';
+import './phosphor/currentselection_test';
diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
new file mode 100644
index 0000000000..af0db27fb2
--- /dev/null
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -0,0 +1,23 @@
+
+import { expect } from 'chai';
+
+import { Selection } from '../../../lib/phosphor/currentselection'
+
+describe('Selection with items', function() {
+    let selection;
+    beforeEach(function() {
+        selection = new Selection(['value-0', 'value-1'])
+        selection.index = null;
+    });
+
+    it('be unselected', function() {
+        expect(selection.index).to.be.null;
+        expect(selection.value).to.be.null;
+    })
+
+    it('set an item', function() {
+        selection.index = 1;
+        expect(selection.index).to.equal(1);
+        expect(selection.value).to.equal('value-1')
+    })
+});

From 1f77604fd811eade0f1a8d33e089ac8dcd67afcb Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Wed, 12 Jul 2017 07:42:27 +0100
Subject: [PATCH 04/24] Test signal triggered correctly

---
 .../test/src/phosphor/currentselection_test.ts  | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index af0db27fb2..5f0f5678f7 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -1,13 +1,18 @@
 
 import { expect } from 'chai';
+import { spy } from 'sinon';
 
 import { Selection } from '../../../lib/phosphor/currentselection'
 
 describe('Selection with items', function() {
     let selection;
+    let subscriber; // subscribe to signals from selection
+
     beforeEach(function() {
         selection = new Selection(['value-0', 'value-1'])
         selection.index = null;
+        subscriber = spy()
+        selection.selectionChanged.connect(subscriber);
     });
 
     it('be unselected', function() {
@@ -20,4 +25,16 @@ describe('Selection with items', function() {
         expect(selection.index).to.equal(1);
         expect(selection.value).to.equal('value-1')
     })
+
+    it('dispatch a signal when setting an item', function() {
+        selection.index = 1;
+        expect(subscriber.calledOnce).to.be.true;
+        const [_, message] = subscriber.getCall(0).args
+        expect(message).to.deep.equal({
+            previousIndex: null,
+            previousValue: null,
+            currentIndex: 1,
+            currentValue: 'value-1'
+        })
+    })
 });

From f481d54fb3dfe7e00926ca0be287b89f34c5eef2 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Wed, 12 Jul 2017 07:50:25 +0100
Subject: [PATCH 05/24] Test for setting by value

---
 .../test/src/phosphor/currentselection_test.ts | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index 5f0f5678f7..43bb5e7592 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -37,4 +37,22 @@ describe('Selection with items', function() {
             currentValue: 'value-1'
         })
     })
+
+    it('set a value', function() {
+        selection.value = 'value-0';
+        expect(selection.value).to.equal('value-0');
+        expect(selection.index).to.equal(0)
+    })
+
+    it('dispatch a signal when setting a value', function() {
+        selection.value = 'value-0';
+        expect(subscriber.calledOnce).to.be.true;
+        const [_, message] = subscriber.getCall(0).args
+        expect(message).to.deep.equal({
+            previousIndex: null,
+            previousValue: null,
+            currentIndex: 0,
+            currentValue: 'value-0'
+        })
+    })
 });

From 52d8e2686711bbf5db6acc53d6b9b38f2f938443 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Wed, 12 Jul 2017 07:55:11 +0100
Subject: [PATCH 06/24] Test for setting index and values to wrong values

---
 .../test/src/phosphor/currentselection_test.ts       | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index 43bb5e7592..8716161787 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -55,4 +55,16 @@ describe('Selection with items', function() {
             currentValue: 'value-0'
         })
     })
+
+    it('set to null if the index is out of bounds', function() {
+        selection.index = 22;
+        expect(selection.index).to.be.null;
+        expect(selection.value).to.be.null;
+    })
+
+    it('set to null if the value is not present', function() {
+        selection.value = 'not-a-value';
+        expect(selection.index).to.be.null;
+        expect(selection.value).to.be.null;
+    })
 });

From 98e48b6588a0ebdaad7897de4942c2975d7b06ad Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 06:46:29 +0100
Subject: [PATCH 07/24] Unittest for inserting an item in selection

---
 .../test/src/phosphor/currentselection_test.ts       | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index 8716161787..9f4982778d 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -7,9 +7,10 @@ import { Selection } from '../../../lib/phosphor/currentselection'
 describe('Selection with items', function() {
     let selection;
     let subscriber; // subscribe to signals from selection
+    const sequence = ['value-0', 'value-1'];
 
     beforeEach(function() {
-        selection = new Selection(['value-0', 'value-1'])
+        selection = new Selection(sequence)
         selection.index = null;
         subscriber = spy()
         selection.selectionChanged.connect(subscriber);
@@ -67,4 +68,13 @@ describe('Selection with items', function() {
         expect(selection.index).to.be.null;
         expect(selection.value).to.be.null;
     })
+
+    it('adjust after inserting an item', function() {
+        const insertedValue = 'value-before-1';
+        sequence.splice(1, 0, insertedValue);
+        // sequence is now [value-0, value-before-1, value-1]
+        selection.adjustSelectionForInsert(1, insertedValue);
+        expect(selection.index).to.equal(1);
+        expect(selection.value).to.equal(insertedValue);
+    });
 });

From 88e4182db2a9fa74d07d5eae1f5c921c7d5b5b57 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 06:49:20 +0100
Subject: [PATCH 08/24] Test for adjusting selection after items moved

---
 .../test/src/phosphor/currentselection_test.ts         | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index 9f4982778d..3659b8c136 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -7,9 +7,10 @@ import { Selection } from '../../../lib/phosphor/currentselection'
 describe('Selection with items', function() {
     let selection;
     let subscriber; // subscribe to signals from selection
-    const sequence = ['value-0', 'value-1'];
+    let sequence;
 
     beforeEach(function() {
+        sequence = ['value-0', 'value-1']
         selection = new Selection(sequence)
         selection.index = null;
         subscriber = spy()
@@ -77,4 +78,11 @@ describe('Selection with items', function() {
         expect(selection.index).to.equal(1);
         expect(selection.value).to.equal(insertedValue);
     });
+
+    it('adjust after an item move', function() {
+        sequence = [sequence[1], sequence[0]]
+        selection.adjustSelectionForMove(0, 1)
+        expect(selection.index).to.be.null
+        expect(selection.value).to.be.null
+    });
 });

From 1a4a0193f77ca6ddf69b24b1d2a72e01ade1f34f Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:05:35 +0100
Subject: [PATCH 09/24] Test for adjusting selection after moving item

---
 .../controls/test/src/phosphor/currentselection_test.ts    | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index 3659b8c136..aebfce81a6 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -85,4 +85,11 @@ describe('Selection with items', function() {
         expect(selection.index).to.be.null
         expect(selection.value).to.be.null
     });
+
+    it('adjust after removing an item', function() {
+        sequence = [sequence[1]];
+        selection.adjustSelectionForRemove(0, 'value-0');
+        expect(selection.index).to.be.null;
+        expect(selection.value).to.be.null;
+    });
 });

From 465c1dd7b71cf7c6bb35640e028fbbd11d4d4b58 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:05:47 +0100
Subject: [PATCH 10/24] Test for clearing selection

---
 .../controls/test/src/phosphor/currentselection_test.ts     | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index aebfce81a6..f61f74efab 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -92,4 +92,10 @@ describe('Selection with items', function() {
         expect(selection.index).to.be.null;
         expect(selection.value).to.be.null;
     });
+
+    it('clear the selection', function() {
+        selection.clearSelection();
+        expect(selection.index).to.be.null;
+        expect(selection.value).to.be.null;
+    });
 });

From a184d2313ba530454a9884f1a7c480a59dc6dfcd Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:15:19 +0100
Subject: [PATCH 11/24] Tests for changing the item selected

---
 .../src/phosphor/currentselection_test.ts     | 45 ++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index f61f74efab..a97fa29e56 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -2,8 +2,15 @@
 import { expect } from 'chai';
 import { spy } from 'sinon';
 
+import { ArrayExt } from '@phosphor/algorithm';
+
 import { Selection } from '../../../lib/phosphor/currentselection'
 
+function getLastMessage(subscriber) {
+    const [_, message] = subscriber.getCall(0).args
+    return message;
+};
+
 describe('Selection with items', function() {
     let selection;
     let subscriber; // subscribe to signals from selection
@@ -31,7 +38,7 @@ describe('Selection with items', function() {
     it('dispatch a signal when setting an item', function() {
         selection.index = 1;
         expect(subscriber.calledOnce).to.be.true;
-        const [_, message] = subscriber.getCall(0).args
+        const message = getLastMessage(subscriber);
         expect(message).to.deep.equal({
             previousIndex: null,
             previousValue: null,
@@ -99,3 +106,39 @@ describe('Selection with items', function() {
         expect(selection.value).to.be.null;
     });
 });
+
+
+describe('Selection with items with an item selected', function() {
+    let selection;
+    let subscriber; // subscribe to signals from selection
+    let sequence;
+
+    beforeEach(function() {
+        sequence = ['value-0', 'value-1']
+        selection = new Selection(sequence)
+        selection.index = 1;
+        subscriber = spy()
+        selection.selectionChanged.connect(subscriber);
+    });
+
+    it('set another item', function() {
+        selection.index = 0;
+        expect(selection.index).to.equal(0);
+        expect(selection.value).to.equal('value-0')
+        const message = getLastMessage(subscriber);
+        expect(message).to.deep.equal({
+            previousIndex: 1,
+            previousValue: 'value-1',
+            currentIndex: 0,
+            currentValue: 'value-0'
+        })
+    });
+
+    it('adjust after inserting an item', function () {
+        const insertedValue = 'value-before-1';
+        ArrayExt.insert(sequence, 1, insertedValue);
+        selection.adjustSelectionForInsert(1, insertedValue);
+        expect(selection.index).to.equal(2);
+        expect(selection.value).to.equal('value-1');
+    });
+});

From 89238bbf2436f14c301c6e1cea1ededbe1c14e03 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:16:21 +0100
Subject: [PATCH 12/24] Use ArrayExt.insert instead of splice.

---
 packages/controls/test/src/phosphor/currentselection_test.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index a97fa29e56..b1c81f06d7 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -4,7 +4,7 @@ import { spy } from 'sinon';
 
 import { ArrayExt } from '@phosphor/algorithm';
 
-import { Selection } from '../../../lib/phosphor/currentselection'
+import { Selection } from '../../../lib/phosphor/currentselection';
 
 function getLastMessage(subscriber) {
     const [_, message] = subscriber.getCall(0).args
@@ -79,7 +79,7 @@ describe('Selection with items', function() {
 
     it('adjust after inserting an item', function() {
         const insertedValue = 'value-before-1';
-        sequence.splice(1, 0, insertedValue);
+        ArrayExt.insert(sequence, 1, insertedValue);
         // sequence is now [value-0, value-before-1, value-1]
         selection.adjustSelectionForInsert(1, insertedValue);
         expect(selection.index).to.equal(1);

From 8bd2304056cb4cacecedc5535f3c20ac46eb8738 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:22:02 +0100
Subject: [PATCH 13/24] Actually modify items in place

---
 packages/controls/test/src/phosphor/currentselection_test.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index b1c81f06d7..c9bd6fe403 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -87,14 +87,14 @@ describe('Selection with items', function() {
     });
 
     it('adjust after an item move', function() {
-        sequence = [sequence[1], sequence[0]]
+        ArrayExt.reverse(sequence)
         selection.adjustSelectionForMove(0, 1)
         expect(selection.index).to.be.null
         expect(selection.value).to.be.null
     });
 
     it('adjust after removing an item', function() {
-        sequence = [sequence[1]];
+        ArrayExt.removeAt(sequence, 0)
         selection.adjustSelectionForRemove(0, 'value-0');
         expect(selection.index).to.be.null;
         expect(selection.value).to.be.null;

From e170bb2affbcae8e056d1774093723fdb8e6241b Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:26:17 +0100
Subject: [PATCH 14/24] Tests for removing an item

---
 .../src/phosphor/currentselection_test.ts     | 21 +++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index c9bd6fe403..e10a79a7fc 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -141,4 +141,25 @@ describe('Selection with items with an item selected', function() {
         expect(selection.index).to.equal(2);
         expect(selection.value).to.equal('value-1');
     });
+
+    it('adjust after removing an item', function() {
+        ArrayExt.removeAt(sequence, 0)
+        selection.adjustSelectionForRemove(0, 'value-0')
+        expect(selection.index).to.equal(0);
+        expect(selection.value).to.equal('value-1');
+    });
+
+    it('adjust after removing the selected item', function() {
+        ArrayExt.removeAt(sequence, 1)
+        selection.adjustSelectionForRemove(1, 'value-1')
+        expect(selection.index).to.equal(0);
+        expect(selection.value).to.equal('value-0');
+        const message = getLastMessage(subscriber);
+        expect(message).to.deep.equal({
+            previousIndex: 1,
+            previousValue: 'value-1',
+            currentIndex: 0,
+            currentValue: 'value-0'
+        });
+    })
 });

From 04c10b04d61ccc81364e3f262edbc5d439a63584 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:28:02 +0100
Subject: [PATCH 15/24] Long line fix

---
 ipywidgets/widgets/widget_selectioncontainer.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/ipywidgets/widgets/widget_selectioncontainer.py b/ipywidgets/widgets/widget_selectioncontainer.py
index 404005ef30..a3e9e95c60 100644
--- a/ipywidgets/widgets/widget_selectioncontainer.py
+++ b/ipywidgets/widgets/widget_selectioncontainer.py
@@ -16,7 +16,9 @@
 class _SelectionContainer(Box, CoreWidget):
     """Base class used to display multiple child widgets."""
     _titles = Dict(help="Titles of the pages").tag(sync=True)
-    selected_index = CInt(help="The index of the selected page.", allow_none=True).tag(sync=True)
+    selected_index = CInt(
+        help="The index of the selected page.", allow_none=True
+    ).tag(sync=True)
 
     # Public methods
     def set_title(self, index, title):

From eac8ae448eb02ba130d41c056409762248785838 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:42:54 +0100
Subject: [PATCH 16/24] Validate that selected index is not out of bounds

---
 .../widgets/tests/test_selectioncontainer.py  | 28 +++++++++++++++++++
 .../widgets/widget_selectioncontainer.py      |  9 +++++-
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 ipywidgets/widgets/tests/test_selectioncontainer.py

diff --git a/ipywidgets/widgets/tests/test_selectioncontainer.py b/ipywidgets/widgets/tests/test_selectioncontainer.py
new file mode 100644
index 0000000000..9b5dee737e
--- /dev/null
+++ b/ipywidgets/widgets/tests/test_selectioncontainer.py
@@ -0,0 +1,28 @@
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+from unittest import TestCase
+
+from traitlets import TraitError
+
+from ipywidgets.widgets import Accordion, HTML
+
+
+class TestAccordion(TestCase):
+
+    def setUp(self):
+        self.children = [HTML('0'), HTML('1')]
+
+    def test_selected_index_none(self):
+        accordion = Accordion(self.children, selected_index=None)
+        state = accordion.get_state()
+        assert state['selected_index'] is None
+
+    def test_selected_index(self):
+        accordion = Accordion(self.children, selected_index=1)
+        state = accordion.get_state()
+        assert state['selected_index'] == 1
+
+    def test_selected_index_out_of_bounds(self):
+        with self.assertRaises(TraitError):
+            Accordion(self.children, selected_index=-1)
diff --git a/ipywidgets/widgets/widget_selectioncontainer.py b/ipywidgets/widgets/widget_selectioncontainer.py
index a3e9e95c60..c3e2314a95 100644
--- a/ipywidgets/widgets/widget_selectioncontainer.py
+++ b/ipywidgets/widgets/widget_selectioncontainer.py
@@ -9,7 +9,7 @@
 
 from .widget_box import Box, register
 from .widget_core import CoreWidget
-from traitlets import Unicode, Dict, CInt
+from traitlets import Unicode, Dict, CInt, TraitError, validate
 from ipython_genutils.py3compat import unicode_type
 
 
@@ -20,6 +20,13 @@ class _SelectionContainer(Box, CoreWidget):
         help="The index of the selected page.", allow_none=True
     ).tag(sync=True)
 
+    @validate('selected_index')
+    def _validated_index(self, proposal):
+        if proposal.value is None or 0 <= proposal.value < len(self.children):
+            return proposal.value
+        else:
+            raise TraitError('Invalid selection: index out of bounds')
+
     # Public methods
     def set_title(self, index, title):
         """Sets the title of a container page.

From e5b298c4140426c49fd9b8bfe12032565e76d523 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:45:16 +0100
Subject: [PATCH 17/24] Appropriate file header

---
 packages/controls/test/src/phosphor/currentselection_test.ts | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packages/controls/test/src/phosphor/currentselection_test.ts b/packages/controls/test/src/phosphor/currentselection_test.ts
index e10a79a7fc..1f0fdd130c 100644
--- a/packages/controls/test/src/phosphor/currentselection_test.ts
+++ b/packages/controls/test/src/phosphor/currentselection_test.ts
@@ -1,3 +1,5 @@
+// Copyright (c) Jupyter Development Team.
+// Distributed under the terms of the Modified BSD License.
 
 import { expect } from 'chai';
 import { spy } from 'sinon';

From 59fcdb4f7f31ce6c164b4cfd6e7df6e20c2f6977 Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 08:02:35 +0100
Subject: [PATCH 18/24] Update documentation for Tabs and Accordions

---
 docs/source/examples/Widget List.ipynb | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/docs/source/examples/Widget List.ipynb b/docs/source/examples/Widget List.ipynb
index 81de14e672..ae09854d85 100644
--- a/docs/source/examples/Widget List.ipynb	
+++ b/docs/source/examples/Widget List.ipynb	
@@ -1071,7 +1071,7 @@
     "\n",
     "Unlike the rest of the widgets discussed earlier, the container widgets `Accordion` and `Tab` update their `selected_index` attribute when the user changes which accordion or tab is selected. That means that you can both see what the user is doing *and* programmatically set what the user sees by setting the value of `selected_index`.\n",
     "\n",
-    "Setting `selected_index = -1` (or any value outside the range of available accordions or tabs) closes all of the accordions or deselects all tabs."
+    "Setting `selected_index = None` (or any value outside the range of available accordions or tabs) closes all of the accordions or deselects all tabs."
    ]
   },
   {
@@ -1100,7 +1100,7 @@
    },
    "outputs": [],
    "source": [
-    "accordion.selected_index = -1"
+    "accordion.selected_index = None"
    ]
   },
   {

From 88ba725882facf963e698171b8d181b58b10436f Mon Sep 17 00:00:00 2001
From: Pascal Bugnion <pascal@bugnion.org>
Date: Thu, 13 Jul 2017 07:51:19 +0100
Subject: [PATCH 19/24] Explain selected_index traitlet

---
 ipywidgets/widgets/widget_selectioncontainer.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/ipywidgets/widgets/widget_selectioncontainer.py b/ipywidgets/widgets/widget_selectioncontainer.py
index c3e2314a95..03ad770151 100644
--- a/ipywidgets/widgets/widget_selectioncontainer.py
+++ b/ipywidgets/widgets/widget_selectioncontainer.py
@@ -17,7 +17,11 @@ class _SelectionContainer(Box, CoreWidget):
     """Base class used to display multiple child widgets."""
     _titles = Dict(help="Titles of the pages").tag(sync=True)
     selected_index = CInt(
-        help="The index of the selected page.", allow_none=True
+        help="""The index of the selected page.
+
+        This is either an integer selecting a particular sub-widget,
+        or None to to have no widgets selected.""",
+        allow_none=True
     ).tag(sync=True)
 
     @validate('selected_index')

From c31ac16581fa65303ee286b9f89fa634edb00de2 Mon Sep 17 00:00:00 2001
From: Jason Grout <jasongrout@users.noreply.github.com>
Date: Fri, 14 Jul 2017 11:03:48 -0500
Subject: [PATCH 20/24] fix typo

---
 ipywidgets/widgets/widget_selectioncontainer.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ipywidgets/widgets/widget_selectioncontainer.py b/ipywidgets/widgets/widget_selectioncontainer.py
index 03ad770151..aa72a081cb 100644
--- a/ipywidgets/widgets/widget_selectioncontainer.py
+++ b/ipywidgets/widgets/widget_selectioncontainer.py
@@ -20,7 +20,7 @@ class _SelectionContainer(Box, CoreWidget):
         help="""The index of the selected page.
 
         This is either an integer selecting a particular sub-widget,
-        or None to to have no widgets selected.""",
+        or None to have no widgets selected.""",
         allow_none=True
     ).tag(sync=True)
 

From a2af8b5b47702f3ed71eb48b1f57e84ea8933ef9 Mon Sep 17 00:00:00 2001
From: Jason Grout <jasongrout@users.noreply.github.com>
Date: Fri, 14 Jul 2017 11:05:32 -0500
Subject: [PATCH 21/24] We don't allow an index out of range now, so update
 docs.

---
 docs/source/examples/Widget List.ipynb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/source/examples/Widget List.ipynb b/docs/source/examples/Widget List.ipynb
index ae09854d85..985d493289 100644
--- a/docs/source/examples/Widget List.ipynb	
+++ b/docs/source/examples/Widget List.ipynb	
@@ -1071,7 +1071,7 @@
     "\n",
     "Unlike the rest of the widgets discussed earlier, the container widgets `Accordion` and `Tab` update their `selected_index` attribute when the user changes which accordion or tab is selected. That means that you can both see what the user is doing *and* programmatically set what the user sees by setting the value of `selected_index`.\n",
     "\n",
-    "Setting `selected_index = None` (or any value outside the range of available accordions or tabs) closes all of the accordions or deselects all tabs."
+    "Setting `selected_index = None` closes all of the accordions or deselects all tabs."
    ]
   },
   {

From cc0d4357cf937c72d201e0f32978160d53d272a7 Mon Sep 17 00:00:00 2001
From: Jason Grout <jasongrout@users.noreply.github.com>
Date: Fri, 14 Jul 2017 11:08:10 -0500
Subject: [PATCH 22/24] Code style

---
 packages/controls/src/phosphor/currentselection.ts | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/packages/controls/src/phosphor/currentselection.ts b/packages/controls/src/phosphor/currentselection.ts
index ce9c24df60..355b63a3f5 100644
--- a/packages/controls/src/phosphor/currentselection.ts
+++ b/packages/controls/src/phosphor/currentselection.ts
@@ -126,8 +126,7 @@ class Selection<T> {
         if (i < 0 || i >= this._array.length) {
             i = null;
         }
-    }
-    else {
+    } else {
       i = null;
     }
 

From 34dd60a87d11ea88e8798f5ba74215282571f9bb Mon Sep 17 00:00:00 2001
From: Jason Grout <jasongrout@users.noreply.github.com>
Date: Sat, 15 Jul 2017 01:19:37 -0500
Subject: [PATCH 23/24] Translate from phosphor convention of -1 being no
 selection

---
 packages/controls/src/phosphor/tabpanel.ts | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/packages/controls/src/phosphor/tabpanel.ts b/packages/controls/src/phosphor/tabpanel.ts
index 0cdc3b8cfa..fe9ecf921f 100644
--- a/packages/controls/src/phosphor/tabpanel.ts
+++ b/packages/controls/src/phosphor/tabpanel.ts
@@ -122,8 +122,10 @@ class TabPanel extends Widget {
    * #### Notes
    * This will be `null` if no tab is selected.
    */
-  get currentIndex(): number {
-    return this.tabBar.currentIndex;
+  get currentIndex(): number | null {
+    const currentIndex = this.tabBar.currentIndex;
+    # Phosphor tab bars have an index of -1 if no tab is selected
+    return (currentIndex === -1 ? null : currentIndex);
   }
 
   /**
@@ -132,8 +134,8 @@ class TabPanel extends Widget {
    * #### Notes
    * If the index is out of range, it will be set to `null`.
    */
-  set currentIndex(value: number) {
-    this.tabBar.currentIndex = value;
+  set currentIndex(value: number | null) {
+    this.tabBar.currentIndex = (value === null ? -1 : value);
   }
 
   /**

From 95cc187c217dee15daf03a4502c6c808b0b9dd75 Mon Sep 17 00:00:00 2001
From: Jason Grout <jasongrout@users.noreply.github.com>
Date: Sat, 15 Jul 2017 01:47:17 -0500
Subject: [PATCH 24/24] Fix js comment

---
 packages/controls/src/phosphor/tabpanel.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/controls/src/phosphor/tabpanel.ts b/packages/controls/src/phosphor/tabpanel.ts
index fe9ecf921f..d44df5d1f8 100644
--- a/packages/controls/src/phosphor/tabpanel.ts
+++ b/packages/controls/src/phosphor/tabpanel.ts
@@ -124,7 +124,7 @@ class TabPanel extends Widget {
    */
   get currentIndex(): number | null {
     const currentIndex = this.tabBar.currentIndex;
-    # Phosphor tab bars have an index of -1 if no tab is selected
+    // Phosphor tab bars have an index of -1 if no tab is selected
     return (currentIndex === -1 ? null : currentIndex);
   }