bugfix: allow zero for fillColumnIndex

fix: Addepar#767

addon/-private/column-tree.js

@@ -12,6 +12,7 @@ import { scheduler, Token } from 'ember-raf-scheduler';
 import { getOrCreate } from './meta-cache';
 import { objectAt, move, splice } from './utils/array';
 import { mergeSort } from './utils/sort';
+import { isEmpty } from '@ember/utils';
 import { getScale, getOuterClientRect, getInnerClientRect } from './utils/element';
 import { MainIndicator, DropIndicator } from './utils/reorder-indicators';
 import { notifyPropertyChange } from './utils/ember';
@@ -653,7 +654,7 @@ export default EmberObject.extend({
       } else if (fillMode === FILL_MODE.LAST_COLUMN) {
         this.resizeColumn(columns.length - 1, delta);
       } else if (fillMode === FILL_MODE.NTH_COLUMN) {
-        assert("fillMode 'nth-column' must have a fillColumnIndex defined", fillColumnIndex);
+        assert("fillMode 'nth-column' must have a fillColumnIndex defined", !isEmpty(fillColumnIndex));
         this.resizeColumn(fillColumnIndex, delta);
       }
     }

tests/integration/components/headers/main-test.js

@@ -167,12 +167,34 @@ module('Integration | header | main', function() {
       );
     });
 
+    test('nth column mode for first column', async function(assert) {
+      let columnWidth = 30;
+
+      await generateTable(this, {
+        fillMode: 'nth-column',
+        fillColumnIndex: 0,
+        widthConstraint: 'eq-container',
+        columnCount: 3,
+        columnOptions: {
+          width: columnWidth,
+        },
+      });
+
+      let tableWidth = table.width;
+      let middleColumnWidth = table.headers.objectAt(0).width;
+
+      assert.ok(
+        Math.abs(tableWidth - middleColumnWidth - 2 * columnWidth) <= 1,
+        'nth column takes extra space in nth column resize mode.'
+      );
+    });
+
     test('nth column mode', async function(assert) {
       let columnWidth = 30;
 
       await generateTable(this, {
         fillMode: 'nth-column',
-        fillColumnIndex: '1',
+        fillColumnIndex: 1,
         widthConstraint: 'eq-container',
         columnCount: 3,
         columnOptions: {