is it possible to paint directly on the image editor but using texture with stencil mode. My object is a cylinder and if i do stencil paint in 3d viewport, i have to go around the surface and because it's curved the texture got stretched/distorted so i think it would be practical just to do the stencil in image editor where the cylinder has been unwarped evenly. But seems i can't do the stencil in image editor , am i correct?
Yes, you can use the stencil in the Image Editor in 2.8 once you confirm you have usable UVs on your object. Good thing is the Cylinder primitive already comes packaged with basic UV mapping. Just be sure to use the Image Editor to paint and not try the UV Editor since they have been split.