> Sorry for the delay. I haven't used master for a while and it doesn't
 > like my init file. I've been spending my time on troubleshooting
 > that. But now I have things mostly up and running.
 >
 > 1. I am currently running master from earlier today which includes
 > your patch from the 23rd (commit
 > fe3af8d4f2a4cd67958f76d1b708e8a78e68cd4f) which I assume is the
 > one you want me to test?

Yes.  I attach it so you can (hopefully) apply it to your Emacs 25
instead of ...

 > 2. I have not applied your proposed definition of
 > `xg_set_geometry' from Message #122
 > (https://debbugs.gnu.org/cgi/bugreport.cgi?bug=25851#122). [But I
 > still have that in my 25.2 rc2 Emacs, which is currently my
 > everyday Emacs.]

... this and test it in your everyday work.

 > 3. I have put
 >
 >    (setq x-gtk-use-window-move t)
 >
 > in the working part of my init file [the rest of my init file --
 > about 10% of it -- is commented out right now].
 >
 > 4. In my desktop file I have three frames specified each with
 > a distinctive size and position.
 >
 > The result is that:
 >
 > - Frames are successfully restored to their former size and
 > position.
 >
 > - No GTK warning messages are being emitted.

OK.

 > However:
 >
 > - When I switch between frames using the window manager Alt-tab
 > window switching, there is a big flash almost every time when an
 > Emacs frame is displayed. I've never seen this problem before.
 > (Occasionally I've seen a single line of the display flicker while
 > scrolling, but I've never seen Emacs make the whole screen flash
 > like this before.)
 >
 > [The flash is too fast to describe other than to say it is light
 > in colour and appears to be horizontal, almost as if there is a
 > white or grey highlight being applied randomly to about half the
 > lines in the frame and then turned off again.]
 >
 > [This was after I'd maximized the three frames so that I could use
 > them, rather than when they were the smaller size at which they
 > were specified in the desktop file I tested with. I tried
 > restoring them down to their previous size and didn't see the
 > flash, then maximised them again and the flashing was back. I
 > suppose that the flashing might also be happening when they're
 > small but that my eye isn't fast enough to detect it; not sure.]
 >
 > Of course this flashing might be unrelated, it might be due to a
 > change elsewhere in master.

I'm sure that Eli's suggestion is correct, so please inhibit double
buffering in your init file.

 > The display antics at startup are slightly different but still
 > there. The sequence seems to be (with three frames in my desktop
 > file with distinctive sizes and positions):
 >
 > a) A largish light-colour-scheme frame is displayed.
 >
 > b) It shrinks but is still a light colour scheme. (Presumably my
 > default font is applied here.)
 >
 > c) It changes to a dark colour scheme. (Presumably my colour
 > scheme is being applied here.)

I don't know what a colour scheme is.  Please send me yours and I'll try
adding it to my .emacs to see which behavior it causes.

BTW I never thought about possible mergings of `initial-frame-alist' or
`default-frame-alist' and the parameter alists applied by desktop.

 > d) It sits there for a bit with the various startup messages
 > appearing in the echo area.
 >
 > e) It disappears and the three frames in my init file appear.
 > (Alternative explanation: It changes its size and position to that
 > of one of the frames in my init file, and the other two frames
 > from my init file are displayed.)

I suppose you mean "desktop" instead of "init" file here?

 > These antics are different from with Emacs 25 where I never see a
 > light-colour-scheme frame unless I start with `-Q'.

To get one aspect right: Do you see the light-colour-scheme frame also
when you don't restore the desktop - with one or three initial frames?
(And what else does master dislike about your init file?)

martin