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From Unreal Wiki, The Unreal Engine Documentation Site

Baseclass for client windows. Clientwindows are used as a container for controls.

Do not subclass from this class directly. Subclass from UWindowDialogClientWindow instead.

Known subclasses

UWindow

• UWindowDialogClientWindow

UMenu

• UMenuHelpClientWindow

UBrowser

• UBrowserInfoClientWindow
• UBrowserMainClientWindow
• UBrowserScreenshotCW