Until
the popover API[
^] is adopted by all major browsers - which is still a loooong way off - an element cannot escape the bounds of it's nearest ancestor
block formatting context[
^].
If you want a custom "drop-down" element to appear outside of the element with
overflow-y:scroll
set, you will need to move the dropdown element outside of the overflow element, either in the markup or by using Javascript.
(You could try using the popover polyfill[^] library; but it would almost certainly be simpler to write a specific script for your particular case instead.)