// For widget/src/windows/nsWindow.cpp, in nsWindow::DispatchMouseEvent

RECT windowRect;
::GetWindowRect(mWnd, &windowRect);
nsIntPoint mousePos;
mousePos.x = windowRect.right/2;
mousePos.y = windowRect.bottom/2;
RECT mouseRect = { mousePos.x, mousePos.y, mousePos.x, mousePos.y };
ClipCursor(&mouseRect); // Forces the mouse to be only within that rectangle on the screen, giving it no width and height means it's locked.