Fix: split_cell doesn't always split cell (#6017)

* Split cells if a single cursor is positioned at the beginning or end of a cell

* Duplicate cell if full cell is selected

* fix null replace

notebook/static/notebook/js/cell.js

@@ -20,6 +20,10 @@ define([
     'services/config',
 ], function($, utils, i18n, CodeMirror, cm_match, cm_closeb, cm_comment, configmod) {
     "use strict";
+
+    function is_single_cursor(dict1, dict2) {
+        return ((dict1.line == dict2.line) && (dict1.ch == dict2.ch));
+    };
     
     var overlayHack = CodeMirror.scrollbarModel.native.prototype.overlayHack;
 
@@ -598,24 +602,44 @@ define([
      * @method get_split_text
      **/
     Cell.prototype.get_split_text = function () {
+        var start = {line:0, ch:0};
+        var last_line_num = this.code_mirror.lineCount()-1;
+        var last_line_len = this.code_mirror.getLine(last_line_num).length;
+        var end = {line:last_line_num, ch:last_line_len};
+
+        var flag_empty_cell = is_single_cursor(start, end);
+        var flag_first_position = false;
+        var flag_last_position = false;
+        var flag_all_select = false;
+
         var ranges = this.code_mirror.listSelections();
 
-        var cursors = [{line: 0, ch: 0}];
+        var cursors = [start];
 
         for (var i = 0; i < ranges.length; i++) {
             // append both to handle selections
-            if (ranges[i].head.sticky == 'before') {
+            // ranges[i].head.sticky is null if ctrl-a select
+            if ((ranges[i].head.sticky == 'before') || (ranges[i].head.sticky === null )) {
                 cursors.push(ranges[i].anchor);
                 cursors.push(ranges[i].head);
+                if (is_single_cursor(ranges[i].anchor, start) &&
+                    is_single_cursor(ranges[i].head, end)) {
+                    flag_all_select = true;
+                }
             } else {
                 cursors.push(ranges[i].head);
                 cursors.push(ranges[i].anchor);
+                if (is_single_cursor(ranges[i].head, start) &&
+                    is_single_cursor(ranges[i].anchor, end)) {
+                    flag_all_select = true;
+                }
+            }
+            // single cursor at beginning or end of cell
+            if (is_single_cursor(ranges[i].head, ranges[i].anchor)) {
+                if (is_single_cursor(ranges[i].head, start)) flag_first_position = true;
+                if (is_single_cursor(ranges[i].head, end)) flag_last_position = true;
             }
         }
-
-        var last_line_num = this.code_mirror.lineCount()-1;
-        var last_line_len = this.code_mirror.getLine(last_line_num).length;
-        var end = {line:last_line_num, ch:last_line_len};
         cursors.push(end);
 
         // Cursors is now sorted, but likely has duplicates due to anchor and head being the same for cursors
@@ -630,11 +654,19 @@ define([
 
         // Split text
         var text_list = [];
+        // Split single cursors at first position
+        if (flag_empty_cell || flag_first_position) text_list.push('');
         for (var i = 1; i < locations.length; i++) {
             var text = this.code_mirror.getRange(locations[i-1], locations[i]);
             text = text.replace(/^\n+/, '').replace(/\n+$/, ''); // removes newlines at beginning and end
             text_list.push(text);
         }
+        // Split single cursors at last position
+        if (flag_last_position) text_list.push('');
+        // Duplicate cell if full cell is selected
+        if ((text_list.length == 1) && flag_all_select && !flag_empty_cell) {
+            text_list = text_list.concat(text_list);
+        }
         return text_list;
     };