An edit to my previous comment: ah, I have discoverd that middle-clicking an icon (tricky with a touchpad . .) will launch a new instance of a launcher, as will ctrl-N (though, on my Mint system, the latter opens the Nemo file manager at an address called 'x-nemo-desktop:///'!). I discover also that right-clicking a launcher with multiple windows open shows a list. I've read also that Plank behaves differently on Elementary, which is the distribution to which it belongs. All in all: sorry if I have hijacked this bug report (though I do think the functions I have described could be made more evident to users).
An edit to my previous comment: ah, I have discoverd that middle-clicking an icon (tricky with a touchpad . .) will launch a new instance of a launcher, as will ctrl-N (though, on my Mint system, the latter opens the Nemo file manager at an address called 'x-nemo- desktop: ///'!). I discover also that right-clicking a launcher with multiple windows open shows a list. I've read also that Plank behaves differently on Elementary, which is the distribution to which it belongs. All in all: sorry if I have hijacked this bug report (though I do think the functions I have described could be made more evident to users).