-S or --chop-long-lines
Causes lines longer than the screen width to be chopped (truncated)
rather than wrapped. That is, the portion of a long line that does
not fit in the screen width is not shown. The default is to wrap
long lines; that is, display the remainder on the next line.
I tried to explain what might be going on here, though this won't result in a way to scroll horizontally with a scrollbar, as you seem to want.
If you are (as others suggest as a solution, although I think you asked about direct output) then redirect output to a somename.log file and open it with Applications>Utilities>Log-Viewer (or from commandline as open somename.log).
It's not in the terminal but I found the log viewer to the one best suited to deal with large amount of output (searching, scrolling, filtering).
I suggest using lnav (easily found), it is a fantastic log and stream viewing tool and by default does single lines with horizontal scrolling. All movements by keyboard keys. You can also easily hide lines matching grep patterns so that you can concentrate on the most relevant data.
I use it almost ALL the time.
-fmessage-length=0set, i still get multi-line warning/error output. and I don't (for now).