SharePoint Site distored view

Hi,

In the last 2 days a user have noticed that Sharepoint has been messing around with how it displays itself.

So, before (Please note that the grey and black box was used purposely to cover the text for security reasons)

After - it's like Sharepoint has bumped over a column to the left. The user can now no longer see the left panel and on the top right, it shows this random white box.

There doesn't seem to be any particular trigger... just clicking from one folder to the next, or right-clicking somewhere, has been enough to cause this issue. If he click on his initials in the top-right corner then it 'resets' itself, but even so, it's a minor pain.

Any ideas?

This issue does not occur for other users and he has also tried different browsers. They have put a background image to the site though but could that affect it?

Many thanks