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# customWindowsToEnterFullScreen(for:on:)

Called when the window is about to enter full-screen mode.

```
@MainActor optional func customWindowsToEnterFullScreen(for window: NSWindow, on screen: NSScreen) -> [NSWindow]?
```

## Parameters

`window`

The window to enter full-screen mode.

`screen`

The display screen on which the window will enter full-screen mode.

## Return Value

An array of windows to use for the animation to full-screen mode for `window`; otherwise `nil`.

## Discussion

This method lets a window delegate customize the animation when the window is about to enter full-screen mode by providing a custom window or windows containing layers or other effects. If you don’t want to perform custom animation, you can omit the implementation of this method, or it can return `nil`.

### Special Considerations

If this method and [`customWindowsToEnterFullScreen(for:)`](/documentation/AppKit/NSWindowDelegate/customWindowsToEnterFullScreen(for:)) are both implemented, this method is called.

---

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