Sure, you can check to see if the widget is clipped by checking its bounds against the clipping area, but what about partial clipping, where 1 pixel is within bounds but 299 happen to be outside? There is no way to clip physics colliders during a raycast. Side by side draggable panels is a tricky one, but my original suggestion stands.
Place a collider covering the entire panel. On that collider receive OnHover(true) event. When such event is received, turn off this collider and enable the neighbour. Since you've disabled this collider, all events will now be falling through to your panel. When you move your mouse over to the neighbour's panel (which is currently covered by a collider you enabled), it will also receive OnHover(true), and the process repeats -- disable it, enable the neighbour's cover.