When the Grid is groupped and showing/hiding columns on client-side, the table layout is broken. More specifically, the cell's colspan does not get adjusted when another cell gets hidden.

To replicate the problem:

1. Create a grid and bind it to data
2. Set the EnableHeaderContextMenu to true
3. Load the page
4. Right-click on a column's header to bring up the contextmenu and click "Group By"
5. Right click on a column's header to bring up the contextmenu again and show/hide a few columns from the "Columns" menu.