notebook: up/down arrow keys move to begin/end of line at top/bottom of cell

The behavior is the following:

* If you press UP while on the first row, you go to the beginning of the line.
* If you press UP again (or were already at the beginning of the line), you
  go to the previous cell.
* If you press DOWN while on the last row, you go to the end of the line.
* If you press DOWN again (or were already at the end of the line), you go
  to the next cell.

This applies to `CodeCell`s and `RawCell`s.

IPython/frontend/html/notebook/static/js/codecell.js

@@ -234,7 +234,7 b' var IPython = (function (IPython) {'
 
     CodeCell.prototype.at_top = function () {
         var cursor = this.code_mirror.getCursor();
-        if (cursor.line === 0) {
+        if (cursor.line === 0 && cursor.ch === 0) {
             return true;
         } else {
             return false;
@@ -244,7 +244,7 b' var IPython = (function (IPython) {'
 
     CodeCell.prototype.at_bottom = function () {
         var cursor = this.code_mirror.getCursor();
-        if (cursor.line === (this.code_mirror.lineCount()-1)) {
+        if (cursor.line === (this.code_mirror.lineCount()-1) && cursor.ch === this.code_mirror.getLine(cursor.line).length) {
             return true;
         } else {
             return false;

IPython/frontend/html/notebook/static/js/textcell.js

@@ -12,6 +12,7 b''
 var IPython = (function (IPython) {
 
     // TextCell base class
+    var key = IPython.utils.keycodes;
 
     var TextCell = function () {
         this.code_mirror_mode = this.code_mirror_mode || 'htmlmixed';
@@ -269,6 +270,36 b' var IPython = (function (IPython) {'
     };
 
 
+    RawCell.prototype.handle_codemirror_keyevent = function (editor, event) {
+        // This method gets called in CodeMirror's onKeyDown/onKeyPress
+        // handlers and is used to provide custom key handling. Its return
+        // value is used to determine if CodeMirror should ignore the event:
+        // true = ignore, false = don't ignore.
+
+        var that = this;
+        if (event.which === key.UPARROW && event.type === 'keydown') {
+            // If we are not at the top, let CM handle the up arrow and
+            // prevent the global keydown handler from handling it.
+            if (!that.at_top()) {
+                event.stop();
+                return false;
+            } else {
+                return true;
+            };
+        } else if (event.which === key.DOWNARROW && event.type === 'keydown') {
+            // If we are not at the bottom, let CM handle the down arrow and
+            // prevent the global keydown handler from handling it.
+            if (!that.at_bottom()) {
+                event.stop();
+                return false;
+            } else {
+                return true;
+            };
+        };
+        return false;
+    };
+
+
     RawCell.prototype.select = function () {
         IPython.Cell.prototype.select.apply(this);
         this.code_mirror.refresh();
@@ -278,7 +309,7 b' var IPython = (function (IPython) {'
 
     RawCell.prototype.at_top = function () {
         var cursor = this.code_mirror.getCursor();
-        if (cursor.line === 0) {
+        if (cursor.line === 0 && cursor.ch === 0) {
             return true;
         } else {
             return false;
@@ -288,7 +319,7 b' var IPython = (function (IPython) {'
 
     RawCell.prototype.at_bottom = function () {
         var cursor = this.code_mirror.getCursor();
-        if (cursor.line === (this.code_mirror.lineCount()-1)) {
+        if (cursor.line === (this.code_mirror.lineCount()-1) && cursor.ch === this.code_mirror.getLine(cursor.line).length) {
             return true;
         } else {
             return false;