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I use a trackball mouse (Kensington USB Orbit), which does not have a scroll wheel. I rely on Middle Emulation to scroll by holding both mouse buttons. Earlier this year (est. May 2021) I noticed Middle Emulation had stopped working on my system. I looked into it recently and found that although I can set the "libinput Middle Emulation Enabled" property to true using:

xinput set-prop "Kensington USB Orbit" "libinput Middle Emulation Enabled" "1"

the property is not being set on startup. I have a 10-libinput.conf which has worked for me in the past. Note the line:

Option       "MiddleEmulation" "true"

I looked at my .local/share/xorg/Xorg.1.log Xorg.1.log and found that the property is being read and recognized on startup of the system:

[    16.196] (**) Option "MiddleEmulation" "true"

So, I believe there is a problem with how MiddleEmulation is being set in libinput, after it is read out of an Xorg conf. The feature is functional when I manually enable it using xinput, so the issue is isolated to the read from the configuration file on startup.

Included a dump of xinput list-props for my device if that's helpful: list-props.txt