bug#53894: 27.2; Clipboard copy -> C-y -> M-y -> Same clipboard copy does not push to kill ring

[Ignacio Casso]

Po Lu <luangruo@yahoo.com> writes:

> LGTM, if Eli's concerns of affecting platforms other than X have also
> been resolved.

It should if you can guarantee that (gui-get-selection 'CLIPBOARD
'TIMESTAMP) returns either nil or a valid timestamp. I will advice it in
my machine to return nil and check that everything still works as
expected, but otherwise I can not test it. I should write some comment
about that too.

Also, I you want to go ahead with the ideas of using text properties for
storing the timestamp, I think I'm going to store a copy of the string
in gui--last-selected-text-clipboard just in case, and not the string
that is actually returned. I will also define functions to set and check
the gui--last-selected-text-clipboard variable in one line, to pollute
less the original code. This way is also easier to implement a check of
the window-system variable if we finally want to make sure to only do
this in X systems.

> I think that this change will require you to complete some copyright
> paperwork.  Could someone tell me if that is indeed the case?

It says that for accumulated changes of more than 15 lines I need to do
so. I don't know if it is actually needed for this but I'll start the
paperwork anyway since I'll probably needed in the future.