I don't recommend to assign something like a popup menu to the left click. It's a bit annoying, in practical. But if you like mzscript, you can make things like (jdesk only):
- popup (or whatever) on double-click. I know, desktop2 allow this, too
- popup on left (right, middle) down and clear on up
- popup on left , then right click (left still down)
- etc,...

If you are interrested on such things and don't know to do, drop me a mail