How Window Position is Determined when using SDL_WINDOWPOS_UNDEFINED

When you pass SDL_WINDOWPOS_UNDEFINED as the x or y argument to SDL_CreateWindow(), you are essentially telling SDL: "I don't have a specific preference for this coordinate; let the system decide."

1SDL_CreateWindow(
2  "Let the OS Decide",
3  // Let OS choose horizontal position
4  SDL_WINDOWPOS_UNDEFINED,
5  // Let OS choose vertical position
6  SDL_WINDOWPOS_UNDEFINED,
7  800, 600,
8  0
9);

SDL passes this lack of specification down to the underlying operating system's window manager. The window manager is the part of the OS responsible for the placement, appearance, and management of application windows (like Windows Desktop Window Manager, macOS WindowServer, or X11/Wayland compositors on Linux).

Platform-Specific Heuristics

How the window manager actually chooses the position is platform-dependent and based on its own internal logic and heuristics, aiming for a good user experience. Common strategies include:

• Cascading: Placing new windows slightly offset from the last opened window.
• Near Cursor: Placing the window close to the current mouse cursor position.
• Remembering Position: Recalling where the user last placed the window for this specific application and opening it there again.
• Avoiding Overlap: Trying to place the window so it doesn't completely obscure important system UI elements like taskbars or docks.
• Multi-Monitor Awareness: Placing the window on the "primary" monitor or the monitor where the user interaction is currently focused.
• Tiling: In tiling window managers (common on Linux), the window manager might automatically assign it a specific tile on the screen.

Because you're letting the system handle it, you don't need to worry about calculating screen dimensions, taskbar sizes, or other complexities to find a "good" default position. Using SDL_WINDOWPOS_UNDEFINED is generally the recommended approach unless you have a specific reason to control the initial placement (like centering it using SDL_WINDOWPOS_CENTERED or placing it at exact coordinates).