The only case I can replicate without issue is when trying to install a game not available for the platform. It's happened in a few other cases, but I can't recall how.
The only way to close the dialog is to hit escape.

This is using the open source radeon driver. Have not tried with the amd provided driver.

Processor Information:
    Vendor:  AuthenticAMD
    CPU Family:  0x10
    CPU Model:  0x4
    CPU Stepping:  0x3
    CPU Type:  0x0
    Speed:  3500 Mhz
    4 logical processors
    4 physical processors
    HyperThreading:  Unsupported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Unsupported
    SSE4a:  Supported
    SSE41:  Unsupported
    SSE42:  Unsupported

Network Information:
    Network Speed:  

Operating System Version:
    Ubuntu 13.10 (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.11.0-12-generic
    X Server Vendor:  The X.Org Foundation
    X Server Release:  11403000
    X Window Manager:  Compiz
    Steam Runtime Version:  steam-runtime-release_2013-09-05

Video Card:
    Driver:  X.Org Gallium 0.4 on AMD BARTS

    Driver Version:  3.0 Mesa 9.2.1
    OpenGL Version: 3.0
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 60 Hz
    VendorID:  0x1002
    DeviceID:  0x6738
    Number of Monitors:  1
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1920 x 1080
    Desktop Resolution: 1920 x 1080
    Primary Display Size: 18.78" x 10.55"  (21.54" diag)
                                            47.7cm x 26.8cm  (54.7cm diag)
    Primary VRAM Not Detected

Sound card:
    Audio device: Realtek ALC662 rev1

Memory:
    RAM:  7985 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_US.UTF-8
    Microphone:  Not set
    Total Hard Disk Space Available:  100665 Mb
    Largest Free Hard Disk Block:  84251 Mb