Problem with using two SDL_Windows on the same form

I posted a while ago about creating two SDL_Windows on the same Delphi form, that nothing would draw to the second window no matter what I did. I’ve done a bit more testing and found a workaround. If I call SDL_SetRenderDrawColor() and SDL_RenderFill(nil) with the second SDL_Window’s renderer selected, then after that I’m able to draw to it. This seems to be a general SDL problem and not a renderer-specific one, since I see the same behavior on D3D, OpenGL and GDI.