When running Steam in Big Picture mode, it always uses the resolution of the primary screen as its "Window Size" setting, even if it is started on another screen.

If the resolution of the primary display is smaller than the one where steam is running, this results in steam being displayed only in part of the screen and it makes mouse input impossible, because the buttons are not where they are displayed.
(screenshot)

I'm using i3 (tiling window manager) on arch 64 bit and nvidia 310.19.

System Information:

Processor Information:
    Vendor:  GenuineIntel
    Speed: 2267 Mhz
    4 logical processors
    3 physical processors
    HyperThreading:  Supported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Supported
    SSE4a:  Unsupported
    SSE41:  Supported
    SSE42:  Supported

Network Information:
    Network Speed:  

Operating System Version:
    "Arch Linux" (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.6.11-1-ARCH
    X Server vendor:  The X.Org Foundation
    X Server release:  11301000

Video Card:
    Driver:  NVIDIA Corporation GeForce GT 330M/PCIe/SSE2

    Driver Version:  3.3.0 NVIDIA 310.19
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 59 Hz
    VendorID:  0x10de
    DeviceID:  0xa29
    Number of Monitors:  2
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1366 x 768
    Desktop Resolution: 3286 x 1080
    Primary Display Size: 13.54" x 7.60"  (15.51" diag)
                                            34.4cm x 19.3cm  (39.4cm diag)
    Primary Bus: PCI Express 16x
    Primary VRAM: 1024 MB
    Supported MSAA Modes:  2x 4x 8x 16x 

Sound card:
    Audio device: Realtek ALC269VB

Memory:
    RAM:  3821 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_US.UTF-8
    Microphone:  Not set
    Total Hard Disk Space Available:  234674 Mb
    Largest Free Hard Disk Block:  30079 Mb

Installed software:

Recent Failure Reports: