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Linux: xrandr dual monitor setup failing at boot

I set up my dual screen configuration in my local ~/.xinitrc file:

HDMI_MAIN=$(xrandr | grep -o HDMI-[0-2])
xrandr --output $HDMI_MAIN --mode 1920x1080 --primary --output DVI-0 --mode 1680x1050 --right-of $HDMI_MAIN &

I use the $HDMI_MAIN variable because the number assigned to the HDMI screen may be 0 or 2. The xrandr command sets up the resolution of both screens and puts the DVI screen as secondary screen at the right of the HDMI one.

This morning, for a reason still unknown, the DVI screen stayed black. To activate it manually, I first had to disable it and then re-execute the second part of the usual command:

$ xrandr --output DVI-0 --off
$ xrandr --output DVI-0 --mode 1680x1050 --right-of HDMI-2

Note that the black screen problem only happened after the start of X. Everything was fine during boot and grub. Also the --verbose option of xrandr did not give me any clue.

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