Use different threshold for (auto)scroll in output

Allow, in particular to switch to scolling for longer input (or disable
it) by still keeping the possibility to manually toggle the output to
scroll.

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

@@ -1242,7 +1242,7 b' var IPython = (function (IPython) {'
         for (var i=0; i<ncells; i++) {
             if (cells[i] instanceof IPython.CodeCell) {
                 cells[i].output_area.expand();
-                cells[i].output_area.scroll_if_long(20);
+                cells[i].output_area.scroll_if_long();
             }
         };
         // this should not be set if the `collapse` key is removed from nbformat

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

@@ -1,5 +1,5 b''
 //----------------------------------------------------------------------------
-//  Copyright (C) 2008-2011  The IPython Development Team
+//  Copyright (C) 2008 The IPython Development Team
 //
 //  Distributed under the terms of the BSD License.  The full license is in
 //  the file COPYING, distributed as part of this software.
@@ -9,11 +9,22 b''
 // OutputArea
 //============================================================================
 
+/**
+ * @module IPython
+ * @namespace IPython
+ * @submodule OutputArea
+ */
 var IPython = (function (IPython) {
     "use strict";
 
     var utils = IPython.utils;
 
+    /**
+     * @class OutputArea
+     *
+     * @constructor
+     */
+
     var OutputArea = function (selector, prompt_area) {
         this.selector = selector;
         this.wrapper = $(selector);
@@ -59,8 +70,18 b' var IPython = (function (IPython) {'
         this.collapse();
     };
 
-
+    /**
+     * Should the OutputArea scroll?
+     * Returns whether the height (in lines) exceeds a threshold.
+     *
+     * @private
+     * @method _should_scroll
+     * @param [lines=100]{Integer}
+     * @return {Bool}
+     *
+     */
     OutputArea.prototype._should_scroll = function (lines) {
+        if (lines <=0 ){ return }
         if (!lines) {
             lines = 100;
         }
@@ -84,7 +105,7 b' var IPython = (function (IPython) {'
             }
             // maybe scroll output,
             // if it's grown large enough and hasn't already been scrolled.
-            if ( !that.scrolled && that._should_scroll()) {
+            if ( !that.scrolled && that._should_scroll(OutputArea.auto_scroll_threshold)) {
                 that.scroll_area();
             }
         });
@@ -143,9 +164,51 b' var IPython = (function (IPython) {'
         this.scrolled = false;
     };
 
-
+    /**
+     * Threshold to trigger autoscroll when the OutputArea is resized,
+     * typically when new outputs are added.
+     *
+     * Behavior is undefined if autoscroll is lower than scroll_threshold,
+     * unless it is < 0 then autoscroll will never be triggerd
+     *
+     * @property auto_scroll_threshold
+     * @type Number
+     * @default 20
+     *
+     **/
+    OutputArea.auto_scroll_threshold = 20;
+
+
+    /**
+     * Defautl value for minimal length for output are to be able to switch to
+     * scroll mode
+     *
+     * @property scroll_threshold
+     * @type Number
+     * @default 20
+     *
+     **/
+    OutputArea.scroll_threshold = 20;
+
+
+    /**
+     * Scroll OutputArea if height supperior than a threshold.
+     *
+     * Treshold is exprimed as a number of lines, fallback to a (configurable) default.
+     *
+     * Negative or null (0) threshold will prevent the OutputArea ever to scroll.
+     *
+     * @method scroll_if_long
+     * @param [lines=20,configurable]{Number}
+     *
+     **/
     OutputArea.prototype.scroll_if_long = function (lines) {
-        if (this._should_scroll(lines)) {
+        var n = lines | OutputArea.scroll_threshold;
+        if(n <= 0){
+            return
+        }
+
+        if (this._should_scroll(n)) {
             // only allow scrolling long-enough output
             this.scroll_area();
         }
@@ -157,7 +220,7 b' var IPython = (function (IPython) {'
             this.unscroll_area();
         } else {
             // only allow scrolling long-enough output
-            this.scroll_if_long(20);
+            this.scroll_if_long();
         }
     };
 
@@ -466,7 +529,7 b' var IPython = (function (IPython) {'
         toinsert.append(latex);
         element.append(toinsert);
     };
-    
+
     OutputArea.prototype.append_raw_input = function (content) {
         var that = this;
         this.expand();