I did a fresh install of Debian 9 XFCE on my desktop computer. Afterwards, I noticed a line running horizontally through the bottom of the screen. I also upgraded my laptop from Debian 8 to Debian 9 and have the same problem. I also installed GNOME on both computers, but it does not have this problem. I'm curious if it has something to do with how XFCE renders to the screen.
As suggested, here is the inxi -b output.
System: Host: debian9-desktop Kernel: 4.9.0-3-amd64 x86_64 (64 bit) Desktop: Xfce 4.12.3 Distro: Debian GNU/Linux 9 (stretch) Machine: Device: desktop Mobo: ASRock model: 970 Extreme3 R2.0 BIOS: American Megatrends v: P1.60 date: 06/05/2014 CPU: Octa core AMD FX-8320 Eight-Core (-MCP-) speed/max: 1400/3500 MHz Graphics: Card: NVIDIA GF119 [GeForce GT 610] Display Server: X.Org 1.19.2 drivers: nouveau (unloaded: modesetting,fbdev,vesa) Resolution: email@example.com GLX Renderer: Gallium 0.4 on NVD9 GLX Version: 3.0 Mesa 13.0.6 Network: Card-1: Qualcomm Atheros AR93xx Wireless Network Adapter driver: ath9k Card-2: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller driver: r8169 Drives: HDD Total Size: 2250.5GB (1.1% used) Info: Processes: 201 Uptime: 1:06 Memory: 826.0/16008.8MB Client: Shell (bash) inxi: 2.3.5