Making the spline tool like inkscape draw bezier curve tool.
I was wondering why synfig automatically creates tangent handles?
Also how about changing the vertex handles and tangent handles shape to squares instead of circles?
Double clicking ends creating of spline.
I also was wondering if you could close a shape like he did with the triangle in the video.

To close a loop, select the line, right click on an end node, and select “loop.”

When it comes to modifying Bezier lines, Inkscape generally has more flexibility. A nice feature Synfig has is the ability to import SVG files. I found Synfig has no problem with importing flat colors and lines/shapes. Just note that you have to consider making good, solid shapes in Inkscape for the animation.

I found out the hard way that simply “making the lines look good” doesn’t cut it when you’re trying to get ready to animate.

To close a loop you should only have to double click what do you think.

Check “link origins” in tools options.