bug#51590: follow-mode is broken with header-line and tab-line

[martin rudalics]

> I would like to stop using "text area" in the doc strings and the user
> manual.

OK.  But can we do that on the development branch please?  I recently
spent some time rewriting the window chapter in the Elisp manual and
just do not feel confident enough to install the necessary changes in a
safe manner on the release branch soon.

> It is a confusing term, more so for me, because it has a very
> specific meaning in the display code.

While a comment like

/* Return the window-relative coordinate of the right edge of display
   area AREA of window W.  ANY_AREA means return the right edge of the
   whole window, to the left of the right fringe of W.  */

may look entirely clear to you, it doesn't to me.  So maybe we could try
to first say what

- a "display area" of a window is and where its "right edge" is located,

- what a "window-relative coordinate" is,

- what the "right edge of the whole window" is, and

- what "left of the right fringe of W" stands for in the presence of
  fringes in and outside of margins or a scroll bar on the left.

Next we could try fixing things like

    int text_area_x, text_area_y, text_area_width, text_area_height;

    window_box (w, TEXT_AREA, &text_area_x, &text_area_y, &text_area_width,
                &text_area_height);

in frame.c and

  int text_area_x, text_area_y, text_area_width, text_area_height;

  window_box (s->w, TEXT_AREA, &text_area_x, &text_area_y,
              &text_area_width, &text_area_height);

in xwidget.c if we want to say that the box size of a window is not the
same as the size of its text area (deduced from you earlier comment).

> So from my POV, the ideal solution is to go through all of those
> places and replace "text area" with "window body" (and if the latter
> is already there, just delete the former).

Agreed.  But after the release, pretty please.

martin