[0]: https://blogs.transparent.com/korean/files/2017/08/Stroke-Or...
ㅅ almost looks like 人 [0] (인) on computers (looks like 1 stroke to beginners), but when handwriting it is 2 strokes. ㅈ is also 2 strokes (personal handwriting aside) and looks rather different.
> I have never seen a native korean write ㅈ/ㅅ as displayed by most fonts, at least in non-formal settings.
Here's[0] someone's history of Korean typography which points out that the ㅅ may come from 훈민정음 Hunminjeongeum [1], which indeed has ㅅ .
Also perhaps you haven't seen middle school kids obsess over handwriting with multicolored pens ;)
[0] https://m.blog.naver.com/PostView.nhn?blogId=designmage&logN...
[1]https://en.wikipedia.org/wiki/Hunminjeongeum_Haerye#/media/F...
Traditionally, one stroke means one movement between the brush touching the paper and lifting off - for example the Chinese character 弓 has three strokes: https://en.wiktionary.org/wiki/%E5%BC%93#/media/File:%E5%BC%...