bug#18546: 24.4.50; active display table

[Drew Adams <drew.adams@oracle.com>]

Not sure whether this is a product bug, a product enhancement request,
or a doc bug.  I'm not very familiar with display-table precedence.

(elisp) `Active Display Table' describes the relation between window
display tables and buffer display tables.  But all it says about the
relation is this:

  The window's display table, if there is one, takes precedence over the
  buffer's display table.

What does that mean?  Does it mean that if there is a window display
table then the buffer display table is completely ignored (shadowed)?
Or does it mean that on an individual character basis the entries in the
window display table shadow the corresponding entries in the buffer
display table?

At a minimum, I think the doc should make the relation clear.

I came across this because of this Unix & Linux Stack-Exchange question:

http://unix.stackexchange.com/questions/138145/window-display-table-and-buffer-display-table-conflict-in-emacs

It states that when whitespace.el is used with pp-c-l.el (my library),
the two do not play well together because the latter uses a window
display table and the former uses buffer display tables.

I'm not sure what to do about this, as I don't really understand the
relation between the two (see above).  I proposed a possible fix (?),
but I cannot repro the problem, so I don't know whether the fix is
useful.

If the answer to my question about what the doc means is that the
window display table completely shadows any buffer display table
for a buffer displayed in that window, then isn't that a bit
primitive?  Is it the best behavior/design?

Again, I really know little about display tables.  Just trying to
understand the user's problem with using these two libraries
together, and to figure out what the best way to fix pp-c-l.el is
for that problem.

In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-08-15 on LEG570
Bzr revision: 117706 rgm@gnu.org-20140815043406-p5hbu97cbm7pulcn
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking 'CFLAGS=-O0 -g3' CPPFLAGS=-DGLYPH_DEBUG=1'