1920x1080 output on 21:9 as 16:9 display compressed to 4:3 space absent video=1920x1080 on kernel cm

Description: Booted with no video-related cmdline parameters, 1920x1080
framebuffer output on two simultaneously connected displays outputs normal 16:9
1920x1080 on 1920x1080 hardware, but compresses 1920x1080 output into a 4:3
space on 2560x1080 hardware configured as 1920x1080 hardware in the display's
setup menu.

To reproduce:
systemd default = multi-user.target
Kernels: 4.17.3 Fedora, 4.16.12 & 4.12.14 openSUSE, 4.9.x Stretch, 4.16.x
Buster, 4.4.x Kubuntu 16.04 LTS
Gfxchips: (2X) ATI HD5450 Cedar and EVGA GT218 [GeForce 8400 GS Rev. 3]
Grub: 0.97 (openSUSE 13.1, 15.0, TW) or Grub 2.02~beta2-36 (Kubuntu)
Screen A: eisa 0x0063 "XVT323SV" on HDMI cable (ATI) native 1920x1080
Screen B: eisa 0x408b "DELL U2913WM" on DVI cable (ATI) native max 2560x1080;
"native" via monitor" menu setting 1920x1080

Screen B produces expected 16:9 1920x1080 output when screen A is not physically
connected. Screen B only responds when KMS is engaged (off during POST and
kernel/initrd loading).

Intel Kaby Lake G4600 with same displays and cables with Grub2-efi (using
load_video & gfxpayload=keep) works as expected without video=1920x1080 on
cmdline with 4.16.12 openSUSE, 4.16.12 Buster, 4.15.23 Kubuntu. Intel Haswell
G3220 with Grub 0.97 on openSUSE 4.17.3 with same cables and displays also works
as expected.

Should I report this at bugs.freedesktop.org?
-- 
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 Team OS/2 ** Reg. Linux User #211409 ** a11y rocks!

Felix Miata  ***  http://fm.no-ip.com/