Subject: bug#31031: 27.0; (elisp) `Position Parameters', floating-point values

From: Drew Adams <[email-masked]>
Date: Mon, 2 Apr 2018 14:56:28 -0700 (PDT)

I'm afraid this reads a bit like gobbledygook, to me.

And if I think I understand it, it doesn't seem to correspond to the
behavior I see.

Of course, the "out" is that it says, "In general, it is not a good idea
to position a frame relative to the right or bottom edge of its
display."  But the only case discussed in that context is an initial/new
frame.  And there is a similar caveat about using not using
floating-point with decorated frames.  But again, it speaks only about
"when CREATING decorated frames. 

The text talks about positioning flush to edges of the "display", which
I'm interpreting as the dominating monitor in the case of multiple
monitors.  (Is that wrong?)

What I see is that the dominating monitor seems to have no effect, so I
wonder what "display" means here.

And in fact using any of the following on an existing frame dominated by
the left monitor or the right monitor sends the frame to the _same_
location: its left edge flush with the left edge of the right monitor:

(modify-frame-parameters nil '((left .   0.0)))
(modify-frame-parameters nil '((left . - 0.0)))
(modify-frame-parameters nil '((left .   1.0)))
(modify-frame-parameters nil '((left . - 1.0)))

(And adding (user-position . t) changes nothing in this regard.)

What am I misunderstanding, here?  Can this text please be made more
clear?  It's not really clear how floating-point values are supposed to
be used or what they are supposed to do.  Dunno whether the behavior I'm
seeing is bugged or the doc is wrong or I'm misunderstanding it.

In GNU Emacs 27.0.50 (build 3, x86_64-w64-mingw32)
 of 2018-03-21
Repository revision: e70d0c9e66d7a8609450b2889869d16aeb0363b5
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install -C 'CFLAGS=-O2 -static -g3''