dgrid OnDemandGrid header and content columns not aligned

[TimDietz]

I have a dgrid (OnDemandGrid, DGridSelection) within a dijit ContentPane (within a BorderContainer) that displays over a map. The grid contains a header row (labels) and the content rows are data from a FeatureLayer. As the map is moved, the server is queried and the data updates and refreshes:

Note that despite the scrollbar, the header and content columns are aligned.

However, I now need to display two similar grids, so I've wrapped the ContentPane within a TabContainer. Everything functions as I need it to, except the content column widths no longer match the header column widths:

I have use the debugger to modify the CSS in every way I can think of, but I've hit a wall.  If anyone has any advice, I'd really appreciate it.

(This is coded within a class method in TypeScript):

const DataGrid = declare([OnDemandGrid, DGridSelection]);
private dataGrid: any;
private dataGridTabContainer: TabContainer;

// columns and store are parameters passed to the method.
columns: { field: string; label: string }[];
store: Memory; // from 'dstore/Memory'

this.dataGrid = new DataGrid(
{
columns,
bufferRows: Infinity,
selectionMode: 'single',
collection: store,
},
'dataGrid'
);
this.dataGrid.startup();

this.dataGridTabContainer = new TabContainer({
style: "height: 100%; width: 100%;"
}, "dataGridTC");

// Note only one tab for now to simplify troubleshooting.
var cp1 = new ContentPane({
title: "Sites",
content: this.dataGrid // Add the grid to the pane
});

this.dataGridTabContainer.addChild(cp1);
this.dataGridTabContainer.startup();