Fixes #259 by checking if the new overlay with-backdrop is actually contained in another overlay with-backdrop. If that's the case, we want to insert the backdrop element into the parent overlay so that it will be in the same stacking context as the new top overlay.
Fixes #259 by checking if the new overlay with-backdrop is actually contained in another overlay with-backdrop. If that's the case, we want to insert the backdrop element into the parent overlay so that it will be in the same stacking context as the new top overlay.