fix: Fixed rollup divider position (#1125)

fixes #1062

Author: bmingles

packages/iris-grid/src/IrisGridRenderer.ts

@@ -178,7 +178,7 @@ class IrisGridRenderer extends GridRenderer {
     state: IrisGridRenderState
   ): void {
     const { metrics, model, theme } = state;
-    const { groupedColumns, columns } = model;
+    const { groupedColumns } = model;
     const { maxY, allColumnWidths, allColumnXs } = metrics;
     if (
       groupedColumns.length === 0 ||
@@ -187,12 +187,16 @@ class IrisGridRenderer extends GridRenderer {
       return;
     }
 
-    const lastGroupedColumn = groupedColumns[groupedColumns.length - 1];
-    const modelIndex = columns.findIndex(
-      c => c.name === lastGroupedColumn.name
-    );
-    const columnX = allColumnXs.get(modelIndex);
-    const columnWidth = allColumnWidths.get(modelIndex);
+    // This assumes that the engine puts the grouped columns at the start of the
+    // model, so model and visible index match for grouped columns. If we ever
+    // add freeze support for tree tables or allow moving the grouped columns,
+    // this assumption may no longer hold true. If such a change occurs, we'll
+    // need to revisit this since a single vertical divider may no longer make
+    // sense.
+    const lastVisibleGroupColumnIndex = groupedColumns.length - 1;
+
+    const columnX = allColumnXs.get(lastVisibleGroupColumnIndex);
+    const columnWidth = allColumnWidths.get(lastVisibleGroupColumnIndex);
     if (columnX == null || columnWidth == null) {
       return;
     }