I saw this too, it just suddenly began with one release in the 1.26 line. Unfortunately I didn't make note of it. I am running a NV Quadro FX 570M, driver 285.27.
Cool VL Viewer 1.26.3 (2) Jan 21 2012 13:12:53 (Cool VL Viewer)
Release Notes
Built with MSVC version 1400
You are at 132584.1, 229349.7, 24.3 in Caliburn located at sim20957.agni.lindenlab.com (216.82.29.236:13000)
Second Life Server 12.01.06.247303
Release Notes: Error fetching server release notes URL.
CPU: Intel(R) Core(TM)2 Duo CPU T9300 @ 2.50GHz (2493.64 MHz)
Memory: 4031 MB
OS Version: Microsoft Windows 7 64-bit Service Pack 1 (Build 7601)
Graphics Card Vendor: NVIDIA Corporation
Graphics Card: Quadro FX 570M/PCI/SSE2
Windows Graphics Driver Version: 8.17.0012.8527
OpenGL Version: 3.3.0
libcurl Version: libcurl/7.20.1 OpenSSL/0.9.8j zlib/1.2.3
J2C Decoder Version: KDU
Audio Driver Version: FMOD version 3.750000
Qt Webkit Version: 4.7.1 (version number hard-coded)
Packets Lost: 0/18411 (0.0%)
I have attached 3 screens showing where the "green" is. I must now put it in quotes, because in the screen shots the areas are not green but a darker shade of blue.
The area seems to be of "fixed" size and place, and just changes with zoom and POV.