@mitu Thanks for the help - you're right I did have a settings override for the core, and I had no idea those settings contained the screen mode as well.
So I had to delete the settings override file for the core, then set my screenmode in runcommand, then save my settings override again. All working now.

Yes, you rock !

it works with this change
indeed in the file /opt/retropie/supplementary/runcommand/runcommand.sh

changed the line : id="$(echo "$line" | awk '{ print $2 }' | grep "([0-9]\{1,\}x[0-9]\{1,\})")" with this line id="$(echo "$line" | awk '{ print $2 }' | grep -o "(0x[a-f0-9]\{1,\})")" And the line : mode_id[1]="$($XRANDR --verbose | grep " connected" | grep -o "([0-9]\{1,\}x[0-9]\{1,\})")" with this line mode_id[1]="$($XRANDR --verbose | grep " connected" | grep -o "(0x[a-f0-9]\{1,\})")"

Now the video mode selection is working great
Many thanks

for NES my CRT likes
-240p screen format
-640x480 render resolution for the screen format
-717x552 (5 x -40 offset) retroarch scaling resolution
-bilinear filtering set to off

For SNES I use a 702x496 retroarch custom scaling resolution with a 12 x -8 offset and bilinear filtering on. Genesis looks pretty good with bilinear filtering on and standard 4:3 resolution