bug#24963: 26.0.50; toggle-frame-maximized broken by double-buffering patch

[Stephen Berman]

0. emacs -Q
1. M-x toggle-frame-maximized
   => frame is maximized
2. M-x toggle-frame-maximized
   => frame remains maximized, echo area displays "You can run the
   command ‘toggle-frame-maximized’ with <M-f10>"
3. M-x toggle-frame-maximized
   => now frame is restored to its prior size

Git bisect says

c29071587c64efb30792bd72248d3c791abd9337 is the first bad commit
commit c29071587c64efb30792bd72248d3c791abd9337
Author: Daniel Colascione <dancol@dancol.org>
Date:   Thu Oct 20 20:34:36 2016 -0700

    Add double-buffering support to reduce flicker

Something in this patch seems to have affected modify-frame-parameters:
in step 1 of the above recipe, the following code is executed (via
set-frame-parameter): 

(modify-frame-parameters nil (list (cons 'fullscreen 'maximized)))

However, subsequently evaluating (frame-parameter nil 'fullscreen)
returns nil, so that in step 2, the frame-parameter is (again) modified,
though the frame is already maximized.  Now, however,  evaluating
(frame-parameter nil 'fullscreen) returns `maximized', so in step 3, the
frame is now demaximized.

I executed modify-frame-parameters with a couple of other frame
parameters, but didn't observe a similar problem.

In GNU Emacs 26.0.50.5 (x86_64-suse-linux-gnu, GTK+ Version 3.14.15)
 of 2016-11-16 built on rosalinde
Repository revision: f63976857b4458bc7712e91d95c04fe2e2ca0106
Windowing system distributor 'The X.Org Foundation', version 11.0.11601000
System Description:     openSUSE 13.2 (Harlequin) (x86_64)