Milton is an open source application that lets you Just Paint.

There are no pixels, you can paint with (almost) infinite detail.
It feels raster-based but it works with vectors.  It is not an image
editor.  It is not a vector graphics editor.  It is a program that
lets you draw, sketch, and paint.  There is no save button, your
work is persistent with unlimited undo.

WWW: https://www.miltonpaint.com/