Created attachment 95171 [details]
Additional dmesg and Xorg logs

When connecting my graphics card to my monitor (BenQ V2400 Eco) via HDMI the behaviour described in the summary can be observed (as early as mode set via KMS). Example pictures of the pink line can be seen in the linked Arch Linux forums thread, the blurred areas are especially visible when viewing text.
Lowering resolution (xrandr --output ... --mode, tested 1280x720, 640x480) does not change the thickness of the line (and doesn't make it disappear either).

The problem goes away when using the proprietary nvidia drivers in connection with the same setup. Additionally I have tested various combinations of drivers (nvidia, nouveau), connectors (HDMI, VGA (via DVI-to-VGA adaptor)) and monitors (BenQ V2400 Eco, Samsung LE32A436). The respective dmesg and Xorg logs are attached.
Note that pink line/blurry areas issue seems to exclusively occur with the following combination: nouveau + HDMI + BenQ V2400 Eco (which happens to be my main setup)

Versions/revisions of relevant software/hardware can be seen in the attached logs. An MMIO trace has been sent to mmio.dumps@gmail.com along with the output of dmesg and lspci -vvv at that time and the VBIOS (dumped via vbtracetool) of my card. The MMIO trace itself contains a marker when the X server was started ("X is up").