“Compose--.” produces an en dash, not an em dash. An em dash is produced by “Compose---”.

Source:

  grep -e DASH /usr/share/X11/locale/*/Compose

▲ mock-possum 4 days ago | parent | next [-]

As it should be. I wish this convention were present across more software, “-“ “- -“ and “- - -“ should be the UI norm for entering proper dashes in text input controls.

	▲	WhyNotHugo 4 days ago \| parent [-]
		Most software handles this fine if you configure your compositor to use a compose key.

▲ fao_ 3 days ago | parent | prev [-]

Whoops, yep that's the one