Render a LONG manual page, for instance :

      M-x manual-entry bash
  
  ALWAYS triggers this bug for me, on a Fedora 34 x86_64 Linux distro,
  fully up-to-date as of 2021-11-11 -

      $ rpm -q emacs
      emacs-27.2-5.fc34.x86_64

  What happens is that sometimes, owing to buffering,
  only PART of a multi-character terminal escape code
  written by the man(1) process is read, so since
  the default man.el is doing fontification on-the-fly,
  sometimes at some point the whole rest of the manual
  page appears in bold, or italic, or underlined font.

  With these changes to man.el, in the attached patch,
  fontification occurs only once ALL man(1) process
  output has been read.

  So the buffer will briefly flash the un-fontified
  contents, then switch to fully-fontified, with
  no missing font transitions .

  This is preferable to the misinterpretation of
  escape codes due to buffering causing missed
  font transitions.