If you have a UI document set up like this
Visual Element covering screen with display set to false
- Visual Element visible in screen with image a background image and border radius of 32 px and Overflow set to hidden
- - Visual Element with overflow set to hidden again
What happens when in play mode in the editor is that the bottom most visual element is visible on the screen, but is respecting the overflow limits of the parent.
Expected behaviour is that nothing is visible at all because the parent object is not visible, but the bottom layer is being shown unexpectedly. Returning Overflow to default setting causes this to stop happening.