ScrollRect and ScrollBar

I'm having the weirdest issue. Screenshot attached.

My scrollbar and scrolling content work great, until the scroll handle gets to the bottom of the scroll bar (circled in red). Then it ignores mouse clicks on it, and I can't click and drag the handle up.

Anyone have any ideas? I'm going mad.

 

More testing: It's refusing to let me click the scroll handle, but only on the lower half of the handle itself. Clicking on the upper half is fine...

 

More testing: It's not the handle, it's actually happening when the scroll handle gets close to the bottom of the scroll bar, and then I can't click any portion of the scroll handle (that has gone too low) and drag it. Can't drag down further either.

 

Solved. Invisible panel was blocking the raycast, UI Debug info showed the culprit. You can see UI debug info by clicking on the event system object in the scene at runtime.

 

I always liked seeing what the Mouse was Over, raycast hits. for example: