[Foomf]

I know Paint.NET, WinSCP and HexChat does this (Microsoft Store).

[suzumer]

Paint.net stopped releasing source code in 2009.

[vxNsr]

But you can still download it from their website for free. They charge for it on the windows store.

[Dwedit]

Spoiler warning, all .NET programs can be decompiled. This includes many Unity games.

[csdvrx]

Is there something like that on Linux that's gtk based but not as complicated as Gimp?

[grey_earthling]

Drawing fits that description, but it isn't really comparable to Krita — the authors describe it as like MS Paint but for Gnome.

Pinta seems similar, but more Xfce-ish.

=> https://maoschanz.github.io/drawing/

=> https://www.pinta-project.com/