This is (kind of) an OpenGL-only function, which should be avoided when SDL2 isn't using OpenGL.
The only alternative that is recommended is SDL_GetRendererOutputSize, which limits this fix to the SDL_Renderer backend. Still, I think it's better than nothing.
I say that SDL_GL_GetDrawableSize is "kind of" OpenGL-only because it does technically work even when SDL2 isn't using OpenGL.
It's just that it becomes a shim to SDL_GetWindowSize, which is not suitable for high-DPI usage because it reflects the size of the window in screen coordinates, not actual pixels, so it really should be avoided when not using OpenGL.
Clownacy
ocornut
Sylvain