Handle some cases when dragging columns

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@eclipsetrading/hypergrid",
-  "version": "4.0.0-alpha.4",
+  "version": "4.0.0-alpha.5",
   "description": "Canvas-based high-performance grid",
   "main": "src/Hypergrid",
   "repository": {

src/behaviors/Behavior.js

@@ -1088,9 +1088,16 @@ var Behavior = Base.extend('Behavior', {
      */
     swapColumns: function(source, target) {
         var columns = this.columns;
-        var tmp = columns[source];
-        columns[source] = columns[target];
-        columns[target] = tmp;
+        var sourceColumn = columns[source];
+        if (sourceColumn === undefined) {
+            return;
+        }
+        var targetColumn = columns[target];
+        columns[source] = targetColumn;
+        if (sourceColumn === undefined) {
+            return;
+        }
+        columns[target] = sourceColumn;
         this.changed();
     },
 

src/features/ColumnMoving.js

@@ -382,7 +382,8 @@ var ColumnMoving = Feature.extend('ColumnMoving', {
         style.borderTop = '1px solid ' + grid.properties.lineColor;
         style.backgroundColor = grid.properties.backgroundColor;
 
-        var startX = grid.renderer.visibleColumns[columnIndex - scrollLeft].left * hdpiRatio;
+        var foundCol = grid.renderer.visibleColumns[columnIndex - scrollLeft];
+        var startX = (foundCol ? foundCol.left : 0) * hdpiRatio;
 
         floatColumnCTX.scale(hdpiRatio, hdpiRatio);
 
@@ -511,7 +512,7 @@ var ColumnMoving = Feature.extend('ColumnMoving', {
         x = Math.max(minX - 15, x);
 
         //am I at my lower bound
-        var atMin = x < minX && dragColumnIndex !== 0;
+        var atMin = x < minX; // && dragColumnIndex !== 0;
 
         //am I at my upper bound
         var atMax = x > maxX;