@meme, yes, it's really weird. I have tested several books and the bug is consistent: clicking on a clip on the dockable pane has no effect when the focus is on the stylesheet while it works fine on the html files (the clip is pasted on the cursor location or selected text).
As I said it's not a big deal as the other methods of pasting clips work as expected.
If nobody else, running windows 7 64-bit, observes this same behaviour, then maybe it's something particular to my machine. When I will install the upcoming new 0.7 beta or final version, I will let you know if this bug remains.
@DiapDealer, I've tried your 64-bit build and it works fine. I haven't noticed any difference in performance compared to 32-bit; including the bug mentioned above.
Anyway, thanks to all the contributors to this great piece of software!