I assumed this was a hard problem — that the copy operation was destroying line boundaries and merging padding with indentation into an ambiguous space blob. Nope. A hex dump (`pbpaste | xxd`) showed real `0a` newlines at every line boundary, and consistent leading whitespace. The entire fix is: strip trailing whitespace, dedent.

  #!/usr/bin/env python3
  import re, sys

  def clean(text):
      text = re.sub(r'\x1b\[[0-9;]*[a-zA-Z]', '', text)     # strip ANSI escapes
      text = re.sub(r'\x1b\][^\x07]*\x07', '', text)         # strip OSC sequences
      text = re.sub(r'[ \t]+$', '', text, flags=re.MULTILINE) # strip trailing whitespace
      lines = text.split('\n')
      while lines and not lines[0].strip(): lines.pop(0)     # drop leading blank lines
      while lines and not lines[-1].strip(): lines.pop()     # drop trailing blank lines
      indent = min((len(l) - len(l.lstrip()) for l in lines if l.strip()), default=0)
      return '\n'.join(l[indent:] for l in lines) + '\n'

  print(clean(sys.stdin.read()), end='')

    pbpaste | ./unfuck-paste
    pbpaste | ./unfuck-paste | pbcopy   # fix in-place on clipboard