bug#71656: secondary selection and mouse-drag-secondary-pasting is broken in 29.3 under wayland

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: John Heidemann <johnh@isi.edu>
>> Date: Wed, 19 Jun 2024 12:33:04 -0700
>> 
>> Since Fedora 40 and emacs-29.3, mouse-drag-secondary-pasting has thrown
>> the error "Could not assert ownership over selection: SECONDARY".
>> 
>> This is in a default Fedora 40 install with the emacs-29.3-6.fc40.x86_64
>> package, running under Wayland.  I think F40 started using native
>> wayland and gtk, which perhaps lost support for secondary selections?
>> 
>> 
>> To reproduce:
>> 
>> start emacs -q
>> 
>> In *scratch* run:
>> 
>> (require 'mouse-copy)
>> (global-set-key [M-down-mouse-1] 'mouse-drag-secondary-pasting)
>> 
>> Then hold down meta and right click and drag over a word.
>> 
>> Expected behavior:
>> 
>> The text you drag over should be copied to the current location.
>> 
>> Actual behavior:
>> error "Could not assert ownership over selection: SECONDARY"
>> and no copying happens.
>> 
>> 
>> I was the original author of mouse-copy.el and I miss its easy
>> plan-9-like copy what I just moused over.
>> 
>> I'm happy to patch its code, but I'd like to know what changed so I can
>> know what I need to work around.
>> 
>> 
>> Thanks for any pointers,
>
> Thanks.  Po Lu, can you please look into and fix this, or help John
> fix it?

John, this error is signaled when the SECONDARY selection is not
implemented by your Wayland compositor.  I don't believe this is any
responsibility of ours, and hence your Lisp code should resort to a
different selection on PGTK systems.