When you are viewing all layers, the lock layer icon should be hidden (like the blue back arrow is), as it can't be used anyway.
What does it look/sound/act like?
It follows the same visibility rules as the blue back arrow (back to all layers).
What is its purpose?
Just to make the UI a bit cleaner by eliminating the random gap between the lock layer button and other UI elements when the blue back arrow isn't present in between them.
Suggest a new feature to be added.
Feature Description
How should it work?
When you are viewing all layers, the lock layer icon should be hidden (like the blue back arrow is), as it can't be used anyway.
What does it look/sound/act like?
It follows the same visibility rules as the blue back arrow (back to all layers).
What is its purpose?
Just to make the UI a bit cleaner by eliminating the random gap between the lock layer button and other UI elements when the blue back arrow isn't present in between them.