io.github.libtmux.PaneCommand.PaneCommand.frame

The start marker is matched as a whole row after trimming, the end marker as the end of one — never by containment, because the echo of the typed line holds both as substrings and must not be mistaken for either. The asymmetry is the shell's: nothing can precede the start marker on its row, since it prints before the command runs, while the command's own last line precedes the end marker whenever it carried no closing newline. That line is output, and is returned as such.