How to maintain Exec output after exit of inner Exec process ?

When into exec mode, execs will print a bunch of things to stdout, when the process exits and returns control to bbt, bbt will wipe the output, how to maintain what was previously outputed ?

Reproduce the issue by entering exec mode, running an inner process, and observing what happens to stdout when control returns to bbt. Trace the exec output handling and determine how existing output is cleared; done means output produced before the inner process exits remains visible afterward.

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