Annoying kill-ring behavior

[Smith_RS]

This will be my last question for a while. It's been a persistent niggle for a 
long time. I was just wondering why this happens and what level of hackery is 
required to make it go away.

SCENARIO #1

- mouse-1-down, move mouse over text, mouse-1-up, verify kill-ring modified 
correctly
- re-position mouse pointer over desired yank location in another frame
- notice that frame is not raised, do a mouse-1-down, mouse-1-up to raise the 
frame and set point at desired yank location
- mouse-2-down, mouse-2-up, observe correct yank behavior

SCENARIO #2

- mouse-1-down, move mouse over text, mouse-1-up, verify kill-ring modified 
correctly
- re-position mouse pointer over desired yank location in another frame
- notice that frame is not raised, do a mouse-1-down, a very small mouse-drag 
of just a few pixels, mouse-1-up to raise the frame
- mouse-2-down, mouse-2-up, observe that nothing is yanked, because there is 
now a zero-length string at the top of the kill-ring

Can someone clue me in as to how I avoid SCENARIO #2?

I want to know how to prevent empty entries from getting into the kill-ring, or 
remove them after the fact. There is obviously a click-vs-drag pixel threshold 
being set somewhere, but I can't see where that's exposed so I can modify it.

And I don't really understand why zero-length strings are allowed in the 
kill-ring at all.

Thanks.