Subject: secondary selection and mouse-drag-secondary-pasting is broken in 29.3 under wayland

From: John Heidemann <[email-masked]>
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,
   -John Heidemannp