The following correspond to window buttons.
They can be enabled/disabled, hooked, and executed:

• window_command_close
• window_command_maximize
• window_command_minimize
• window_command_restore

The following correspond to window operations.
They can only be enabled/disabled:

• window_command_resize
• window_command_move

Enables/disables the specified command.

Returns whether successful (can only fail if the DLL isn't loaded).

For window buttons, this grays them out.
Operations are just prevented from starting.

For example, the following disables the Close button:

window_command_set_active(window_command_close, false);

Returns whether the specified command is currently enabled.

Force-executes a command.
This can be done even when a command is disabled or hooked.

For example, the following minimizes the window:

window_command_run(window_command_minimize);

On the technical side, this performs a SendMessage(hwnd, WM_SYSCOMMAND, command, lParam) so you can also use it for other messages if you need to - for example, the following would enable the scrensaver:

window_command_run(0xF140);

Returns LRESULT from SendMessageW (generally not very useful).

Hooks the specified command-button.

Returns whether successful (can only fail if the DLL isn't loaded).

Hooked buttons remain enabled and can be pressed by the user, but the action will be intercepted.

For example,
Create:

window_command_hook(window_command_close);

Step:

if (window_command_check(window_command_close)) {
    if (show_question("Really quit?")) game_end();
}

Unhooks the specified command-button, returning it to normal operation.

Returns whether successful (can only fail if the DLL isn't loaded).

Returns whether a command has been triggered by the user since this function was last called for it.

If you have multiple spots in game code check for a command, store it in a global variable in one place and check for it in the rest.

Changes whether the WinAPI will clear the window with background color, which is separate from game background being cleared during normal drawing.

Returns whether successful (can only fail if the DLL isn't loaded).

I don't remember who I added this function for / what the use case was.

Returns the current state for above.

Changes whether the window will stick on top of other windows, akin to a long-forgotten GM8 function.

Returns whether a window is currently stay-on-top.

Changes whether the game window will have a button shown for it in the taskbar.

The current approach is best suitable for borderless windows.

Returns whether successful (can only fail if the DLL isn't loaded).

Returns whether the game window currently has a taskbar button shown for it.

Changes whether the mouse clicks pass right through the game window.

Returns whether successful (can only fail if the DLL isn't loaded).

Returns whether the mouse clicks pass right through the game window.

Changes whether the window is prevented from gaining focus.

Returns whether successful (can only fail if the DLL isn't loaded).

Returns whether the window is prevented from gaining focus.

Changes visibility of the game window, much like the GM8 function.

Don't forget to make your window visible later or call game_end(), else the user will have to look for it in the Task Manager.