Lightroom :: Quick Develop Module Doesn't Show With Cursor Movement?

Somehow, the little triangle that opens and closes the Quick Develop Module in my Library has become solid, so it no longer pops the menu open when I glide my cursor over to the right side of a full screen image. How do I get the Quick develop module triangle to have that shadow look again so that when I glide the cursor to it the module opens.
 
I must have changed this with a keystroke or something but for the life of me I cannot get it back and it's driving me crazy. I do not like the module open all the time and it's a pain to use the keystroke or click the triangle each time I need to make a quick adjustment. I hope this is not an upgrade change. I notice it is the same in my develop module.