If you load Steam with two monitors and then disable the secondary one (or if it was a laptop, remove it) when TF2 loads it'll stretch the screen so you will only be able to see of the game.

To fix it, you have to restart Steam.

Processor Information:
    Vendor:  GenuineIntel
    Speed: 3000 Mhz
    2 logical processors
    2 physical processors
    HyperThreading:  Unsupported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Supported
    SSE4a:  Unsupported
    SSE41:  Supported
    SSE42:  Unsupported

Network Information:
    Network Speed:  

Operating System Version:
    Ubuntu 12.04.2 LTS (32 bit)
    Kernel Name:  Linux
    Kernel Version:  3.2.0-38-generic-pae
    X Server Vendor:  The X.Org Foundation
    X Server Release:  11103000
    X Window Manager:  Compiz
    Steam Runtime Version:  steam-runtime-release-i386_2013-03-08

Video Card:
    Driver:  ATI Technologies Inc. ATI Radeon HD 4800 Series         

    Driver Version:  3.3.11672 Compatibility Profile Context
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 60 Hz
    VendorID:  0x1002
    DeviceID:  0x9442
    Number of Monitors:  2
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1280 x 1024
    Desktop Resolution: 2560 x 1024
    Primary Display Size: 13.31" x 10.67"  (17.05" diag)
                                            33.8cm x 27.1cm  (43.3cm diag)
    Primary VRAM Not Detected

Sound card:
    Audio device: Realtek ALC889A

Memory:
    RAM:  4026 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_GB.UTF-8
    Microphone:  Not set
    Total Hard Disk Space Available:  63084 Mb
    Largest Free Hard Disk Block:  14388 Mb

Installed software:

Recent Failure Reports: