gui-set-selection does not set Emacs' internal clipboard

[R. Diez]

Hi all:

I have recently upgraded to Emacs 29.1, but I do not know whether I am hitting 
a new issue or regression, as I have always had Clipboard problems. Today I am 
trying to understand and fix one specific issue with gui-set-selection.

I have compiled Emacs myself on Ubuntu MATE 22.04.3.

I want Emacs' clipboard to behave like Microsoft Windows, GEdit, Pluma, usw., 
so I have set these variables:

(setq select-enable-clipboard t)
(setq select-active-regions nil)

Say I select and copy a piece of text in Emacs. Then I can paste the text both 
in Emacs and in an external editor like GEdit or Pluma. So far so good.

I have written an Emacs routine which converts something and places it in the X 
clipboard like this:

(gui-set-selection 'CLIPBOARD result)

Say I type, select and copy the piece of text "manual-text" in Emacs, and I then run my 
Emacs routine, which ends up placing in the clipboard the piece of text 
"from-gui-set-selection".

If I now paste inside Emacs, I get "manual-text". However, if I paste inside GEdit, 
Pluma, etc., I get "from-gui-set-selection".

That is, gui-set-selection is not updating Emacs' internal clipboard. Or 
perhaps Emacs does not realise that the X clipboard has changed when it was 
Emacs itself the one which just changed it.

If I now select a piece of text in GEdit, Pluma, etc. and copy it, then I can 
paste it into Emacs without trouble. Well, mostly, but that is an issue for 
another day.

I tried calling gui-set-selection again in my Emacs routine, but this time 
passing nil, 'PRIMARY or 'SECONDARY, to no avail. I must admit that I do not 
really understand how Emacs handles the clipboard.

Can somebody help?

Thanks in advance,
  rdiez