Paint and fill is, as you probably already are aware, coming fairly soon. My plan is to have paint layers, so you can have your line on top of your painted areas. Also, technically, the two kinds of layers are quite different, so there are many good reasons to let them stay separate. Of course when you fill, it will respect the boundaries of the lines in what ever layers you choose. We have plenty of ideas and plans for how it’s all going to work. And about half of the work have already been done.
But I would love to understand better what exactly it is you are asking. I mean, you want a fill tool, that fills behind the line in the same layer. Understood. But why? Let me know why this is important to you? I need to understand your thoughts and workflow, to decide if I need to change anything in my plans. 😉 Please explain. Thanks!