Tom 
I've already assumed that my touch pad was broken (or the driver was buggy) because after a tap-and-drag action, the mouse button was not released when releasing the finger from the touch pad.
It was "sticky", e.g. after marking multiple lines of text.
Just found out that this was introduced a while ago in Sway (to follow libinput recommendations):
https://github.com/swaywm/sway/commit/bbadf9b8b10d171a6d5196da7716ea50ee7a6062
Setting "drag_lock disabled" in the touch pad's config fixes this. 😌
