SDL_WINDOWID hack renders from screen's offsets, not target window's


On Windows, I am using SDL_WINDOWID hack to integrate SDL with another gui
window. However, the SDL portion is rendered from the computer screen’s
upper left-most point, not from the target window’s upper left-most point.

I use "SDL_UpdateRect(my_Surface, OFFSET_X, OFFSET_Y, width, height);"

to update the targeted rectangle inside a window that SDL utilizes from
SDL_WINDOWID. My intention is that the updated portion should be at the
offsets in the window. But the result is that the updated portion is at
(OFFSET_X, OFFSET_Y) of the computer physical screen. How could that happen
and how to solve the issue?