inexplicable fallback mode used when preferred mode 2560x1080 not supported by connection

[Felix Miata]

Since acquiring my 2560x1080 U2913WM Dell for use among several multiboot PCs, 
with AMD/ATI, Intel and NVidia gfxchips represented, I'm frequently annoyed by 
lack of support for native mode by the cable actually connected, not so much by 
the lack of support itself as by the mode fallen back to. In most cases, the 
fallback selected is either 1400x1050 at 75 or 1152x864 at 75, while if using a VGA 
connection, 2560x1080 is supported for every PC and gfxchip I've tried so far.

http://fm.no-ip.com/Tmp/Linux/Xorg/xorg.0.log-big31-ostw-d2913wm.txt is 
Xorg.0.log plus output from inxi -c0 -G, hwinfo --gfxcard and xrandr from a 
GT210[1] on host big31 booted to openSUSE Tumbleweed and modesetting driver, 
which falls to 1400x1050 using an HDMI cable. Using instead nouveau, fall is to 
1152x864, same as same PC booted to openSUSE 42.1 using HDMI and modesetting 
driver, and same PC booted to Fedora 25 using HDMI and nouveau driver.

Obviously the display isn't likely capable of any less than 1920x1080 via HDMI, 
because Intel gfx host big41 booted to Debian does it:
http://fm.no-ip.com/Tmp/Linux/Xorg/xorg.0.log-big41-deb0-d2913wm.txt
OTOH, this installation when I try to use xrandr in startup script to get 
2560x1080 via HDMI also produces instead 1152x864. :-(

Why isn't the fallback only to 1920x1080 instead of all the way back to a much 
less appropriate and anachronistic 4:3 mode? How can I determine whether the 
fault is driver(s), Xorg, Display or Gfxchip? Is an Xorg bug filing at 
freedesktop.org indicated here? Could more elaborate xrandr on startup or 
xorg.conf produce 1920x1080 in these cases?

[1] http://www.evga.com/products/product.aspx?pn=01g-p3-1312-lr
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