I have put both a Modal and a Side Slide in the sidebar area to the left. I am playing off of the "sticky sidebar" example you provided yesterday. In real life I'd only use one. Probably best to try the modal first. If you are at the top of the page when you click the button all is fine. Scroll down a bit and try to use the modal and you'll see the overlapping conflict.
As for the Side Slide it just seems to go nuts. However, again, at the top of the page it works fine. Scroll down a wee bit then use it in the Sticky sidebar and all goes nuts. Essentially the page freezes and I need to refresh to get things going.
So the bad effect is different in the two cases. Although in both cases you need to scroll down a wee bit to see the bad effect.
Here's an example page:
BTW this seems to happen if I only have one of them on a page (instead of both).