Using the Xorg that comes with the antiX 13.1 Linux distro (name probably predestined to have problem with X, ahem), namely X.Org X Server version 1.12.4 (2012-08-27) I see exactly only the left half of the screen, regardless of resolution. Nothing more gets sent to the monitor, I've adjusted it (it's CRT). As expected from the driver (s3), this is on totally ancient hardware. S3 968 (Diamond Stealth 64) with IBM RAMDAC identified as model 526 by X (9314 printed on the chip) with 4 MB VRAM (2+2 on daughter-board card). I've not used this computer in over a decade, and back in the the day of RedHat 6.2 Xfree used to work fine on it... Let me know if you need more info.
After further investigation, it seems Xorg overclocks the card somehow. The monitor is a Samsung 757NF, which is of considerably newer vintage, so it supports better refresh than this card can dish (with its Windows drivers anyway). According the video card's manual (which I managed to dig up in the meantime), the max vertical frequency at 1024x768 is supposed to be 80Hz and at 1280x1024 it's supposed to max out at 75Hz. This is with 16m colors. The data from the card's manual corresponds to what the Windows98(SE) drivers can do and what the monitor reports with that OS booted. (I wasn't able to tell if Windows uses 24 or 32 bpp.) With Xorg however I get a whopping 107Hz (reported by the monitor) at 1024x768 with 32 bpp, and no higher resolution is available with xrandr. Based on my eyeometer, the refresh data reported by the monitor is probably accurate. The image looks a lot more flicker-free than in does Windows. But of course each pixel is distorted and I only get the left half of them. (I'm one of those people who can see flicker up to 85Hz or so. That doesn't seem to go with the age...) The separate hard drive which had RedHat 6.2 (and Xfree) unfortunately caught the click of death, so I can't check that out anymore. This is a PCI card by the way (didn't even know there were Stealth64 VLB cards name until I read the manual), and it's probably the 3240 model given that it has a 2 MB daughterboard. The memory get correctly sized by Xorg. The only thing that looks suspicious in the log is that Xorg can't read the card's BIOS via PCI for some reason.
After further investigation, which involved slapping Windows XP on the ancient box, it turns out that the refresh rate "issue" is not related to the half-screen-visible bug. It seems that the card's manual and Win98 drivers were unnecessarily conservative with the refresh rate. The XP s3 legacy driver puts a 100 Hz at 1024x768 32 bpp with no ill effects.
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