I have this problem since I upgrade to 16.04, it was fine in 14.04.
In panel preference, if I set panel style to xfce 4.12, the panel looks like this.

By default, it's set to Xubuntu modern and it doesn't display steam tray icon correctly.

Processor Information:
    Vendor:  GenuineIntel
    CPU Family:  0x6
    CPU Model:  0x3c
    CPU Stepping:  0x3
    CPU Type:  0x0
    Speed:  3700 Mhz
    4 logical processors
    2 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:
    Ubuntu Xenial Xerus (development branch) (64 bit)
    Kernel Name:  Linux
    Kernel Version:  4.3.0-2-generic
    X Server Vendor:  The X.Org Foundation
    X Server Release:  11703000
    X Window Manager:  Xfwm4
    Steam Runtime Version:  steam-runtime-release_2015-06-12

Video Card:
    Driver:  NVIDIA Corporation GeForce GT 630/PCIe/SSE2

    Driver Version:  4.5.0 NVIDIA 352.63
    OpenGL Version: 4.5
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 60 Hz
    VendorID:  0x10de
    DeviceID:  0x1284
    Number of Monitors:  1
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1280 x 1024
    Desktop Resolution: 1280 x 1024
    Primary Display Size: 13.27" x 10.63"  (16.97" diag)
                                            33.7cm x 27.0cm  (43.1cm diag)
    Primary Bus: PCI Express 8x
    Primary VRAM: 1024 MB
    Supported MSAA Modes:  2x 4x 8x 16x 

Sound card:
    Audio device: Realtek ALC887-VD

Memory:
    RAM:  7947 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_US.UTF-8
    Microphone:  Not set
    Total Hard Disk Space Available:  106151 Mb
    Largest Free Hard Disk Block:  61354 Mb

Installed software:

Recent Failure Reports: