1. install a clipboard manager such as Glipper
2. select a few words of text with the mouse
3. look at the clipboard history and notice that there's an entry for every letter, wiping out most/all previous history
I don't know how the select-text-to-copy-it mechanism works internally, but since there's not really any way for the user to paste before mouseup anyway, webkit probably shouldn't report to the system that text is to be copied until mouseup. Compare to, say, gnome-terminal's behavior, where selecting several words of text in the same way produces only a single clipboard history entry.
Note also that this happens when unselecting text before a mouseup as well (i.e. highlight a word, then drag back to unhighlight it before letting go).
I can't see any such problem with xfce4-clipman. It always inserts exactly one entry for the whole text I selected.