After resizing the columns, even when there are columns outside of the viewport, there is no horizontal scrollbar.
Can't repro this. Can you include repro steps or a specific example?
Go to the simple example:
Increase the size of the column "Duration" by dragging the right edge of its header until the column "% Complete" is hidden.
The horizontal scrollbar should appear now, but it doesn't.
I got the same result in Firefox, IE and Chrome.
Ah, this was only happening in the first example. Fixed.