I've used Photoshop since before dirt was invented. Right now I'm using PS CC v19.
I must have flipped a switch somewhere because now it won't let me paint on a layer or fill a selection. I have no idea why.
This occurs with existing files and new files alike and persists even if I close PS and open try anything else. (It has nothing to do with the file in my screenshot; it was a problem before and it's a problem after. And it's a problem with any file)
I know I did some unusual things in PS I've never done before - experimenting with converting a bitmap to a .STL file, and using the "3D" options form the menu, so PS may have "helped" me by switching to some mode I'm not aware of.
This is a normal RGB, 8-bits/channel mode, just like always.
They both have a new (indecipherable) icon next to them: