as a side effect, i am a little bit worry about the UI/UX of Synfig Sutdio, why a user has to take this kind of complex steps to do such a simple thing. “Draw lines (outline) to form a shape, fill it” should be intuition, instead of asking user to take a lot of effort to figure it out. It may worthy to implement a Real Fill tool and place it into toolbox.
Maybe possible if the user clicks specifically on the outline to fill (not just inside of it).
EDIT: Well, seems it might be possible to have a fill tool which works by clicking inside an outline layer. But it still not a real flood fill tool, it still would rely on creating the new region layer using the same spline as the outline layer. So it would not possible to, for example, create a circle outline, split it in half with another outline and fill each side with a different color.