"Drag" is an action idiom common to several GUI canons, roughly co-ordinate with, say "pan". It most often appears in discussions of drag and drop facilities.
It only takes a few lines of Tk to exhibit dragging. Here's an example Michael Kraus offered in the comp.lang.tcl newsgroup:
pack [button .b -text "The Button" -command exit] wm overrideredirect . 1 bind .b <1> { set iX0 [expr %X-[winfo rootx .b]] set iY0 [expr %Y-[winfo rooty .b]] set bMoved 0 } bind .b <B1-Motion> { wm geometry . +[expr %X-$iX0]+[expr %Y-$iY0] set bMoved 1 } bind .b <ButtonRelease-1> { if { $bMoved } break }
This [describe effect ...]
[Next up: exhibit dragging an item around a canvas.]