Top-level output records are similar to ordinary output records, except that they maintain additional state, such as the information required to display scroll bars.
Summary: This class is mixed into some other output record class to produce a new class that is suitable for use as a a top-level output history.
When the bounding rectangle of a member of this class is updated, any window decorations (such as scroll bars) associated with the stream with which the output record history is associated are updated, too.
Summary: The standard class provided by CLIM to use as the top-level output history. This will typically be a subclass of both standard-tree-output-record and stream-output-history-mixin .
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017