Dunno what I've done but was messing around with the toolbar thing and now when I try to do anything on the painting it pops up and says " Selection prevents painting " and I can't do anything.

I've gone into the Layers tool to make sure the layers weren't locked and they are not.

What have I done and how do I undo it?

==============
Nevermind

I discovered I had added stickers of a couple of gulls and I found out I had to flatten them to paint!!
Once I did that it let me resume smearing stuff on the paper.

But, I have a new question. It seems to me that in the distant past I knew a shortcut in the layers panel that would make only the current layer visible but I can't remember what it was.

Is there still such a shortcut?

Sometimes I just want to one layer and now I have to go to each one and make it disappear.
I know when it is yellow the focus is on that layer and the paint only applies to that layer, but I want it so that layer is all I see.