Hello,
It seems that support of "--output" by meld is performed by grepping its help output. However with meld 3.12 I get
$ meld --help Usage: meld [OPTION...]
Help Options: -h, --help Show help options --help-all Show all help options --help-gtk Show GTK+ Options --help-gapplication Show GApplication options
Application Options: --display=DISPLAY X display to use
Yet it still supports --output and can show nice middle instead of <<<<<< HEAD etc. markers.
Thank you, Andrey