scroll box inside retainer box does not respect scroll box clipping

There is an issue where scroll boxes inside retainer boxes do not respect scroll box clipping. This issue appears to be a regression.

Versions tested: Issue does not occur in 4.17
4.17.2 CL - 3658906
4.18.3 CL - 3832480
4.19p4 CL - 3873914
4.20.0 CL - 3879785

1. Create a new project (no template or starter content needed)
2. Create a new widget blueprint > name it "TestBP" > open TestBP
3. Add a retainer box to the canvas panel and set the size to take up all but a small part of the bottom of the canvas panel space
4. Add a scrollbox as a child of the retainer box
5. Add a button as a child of the scroll box
6. In the details panel of the button set the size of normal and hovered to 400 on the Y axis
7. Set the color of the hovered state of each button to a new color
8. Compile > save > and close the blueprint
9. In the level blueprint > create the widget "TestBP" > add it to the viewport
10. Get a reference to the player controller and set the mouse to show
11. PIE
12. move the mouse so it hovers the bottom button then move the mouse off of the widget
13. Result: The button is still colored as if it hovers
14. Expected Result: The button is no longer hovered and colored