Your system information

• Steam client version: Steam client beta (2016-06-29)
• Distribution: lubuntu 14.04
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

Run Left 4 Dead 2 in "Window without border" mode, the lowest part is covered by lubuntu taskbar
Expected: Game window is on top of everything created by X window
What happened: Lubuntu taskbar is on top of game window, covered lowest part of the game window

Steps for reproducing this issue:

1. Set lubuntu resolution to 1920 x 1080
2. Run Left 4 Dead 2
3. In Left 4 Dead 2 "Video" config page, set resolution to 1920 x 1080 and "Window without border" mode