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While running drawterm under sway (wayland), the middle mouse button triggered both down and up only *after* the button was released (as seen by xev).
It came down to 'button scrolling' on the TrackPoint device (you can hold the middle button and scroll with the trackpoint). I couldn't figure out how to get libinput to stop doing this.
With a nudge from @sigrid@merveilles.town toward sway-input and it's man page. I added the following to my sway config:
input * scroll_button disable
