In Windows NT, applications keep track of the viewer chain through this function and ChangeClipboardChain. In OS/2, OS/2 keeps track of the viewer chain.
In OS/2, only one viewer is allowed and OS/2 performs no arbitration when an application sets itself as the viewer; OS/2 just ends the old viewer with no warning. Each application thinks that it is the only viewer in the system.
In OS/2, SetClipboardViewer always returns 0. OS/2 does not tell applications about other viewers. In OS/2, it is not the application's responsibility to pass clipboard messages on to the next viewer in the chain. In Windows NT, 0 indicates there is no next viewer.
In OS/2, applications are never sent WM_CHANGECBCHAIN; this message is meaningless in OS/2. In Windows NT, the viewer that is leaving the chain does not get this message; in OS/2, the other viewers do not know about each other so this message is meaningless to them. In Windows NT, they would just pass it on.