Grid headers do not scroll after Grid#reconfigure()

[tby]

Reproduction:
1. Build a Grid which is wider than your browser window.
2. Reconfigure the Grid:

grid.reconfigure(grid.getStore(), grid.getColumnModel());

3. Scroll to the left in the Grid with the mouse.

Expected:
The headers scroll along with the grid.

Actual:
The headers are fixed. Only the grid content scrolls.