can be tested by installing our last iteration of fr-FR pack https://github.com/infograf768/fr-FR-3.x/releases/download/v3.7.0.1.RC3/fr-FR_joomla_lang_full_3.7.0v1.zip

@infograf768 - already tried that and getting a JS error from $.ajax on the installer

will test again later on. It was working OK a few days ago.

No problem here to install lang. Will now test this PR

Works. Does not solve #15439

@infograf768 this PR just removes a totally unneeded dependency (jQuery), the other issue is due to multiselect

There is a difference though with 3.7.0 (before and after this PR)
When one loads an existing associated item, The Copy Reference to target is proposed in the side by side, which is not the case in 3.7.0 (3.7.0 is correct).
Basically Copy Reference to Target should only display and be used when the target is a new empty item.

Example:

instead of

I have displayed above on top what is right : 3.7.0 : NO Copy ref to target
below what is wrong: 4.0 Copy to Target is present
In both cases for items which alre already associated

as any one can see at looking at the screenshots
Suggest ? ?

Sorry, just removed my comment as I just realised the 2nd screenshot was Joomla 4 :)

As for the button in the toolbar, I don't believe that has anything to do with this PR. It may need to be fixed, but still unrelated.

The code to display or not the Copy is in the sidebyside.js and DOES need to be fixed (why does'nt it work in 4.0 is a mystery to me)
I has indeed apparently no relation with this PR. Therefore this can be merged.

Quote from my initial PR comment:

I'm not going to event attempt to rewrite sidebyside.js as it's an absolute cluster***k

@infograf768 javascript is tightly connected to the markup, and that is slightly different is J4

I understand that. I also saw that the Save Target button is displayed even when target is not even displayed yet.
we really need to solve these .

Yeah, the markup indeed...
in 3.7.0 we have a

<div id="toolbar-target" class="btn-wrapper" style="display: inline-block;">
     <button class="btn btn-small btn-success" onclick="Joomla.submitbutton('target')">
etc.

jQuery(document).ready(function($) {
	$('#toolbar-target').hide();
	$('#toolbar-copy').hide();

We do need ids for these toolbar buttons.
Can't we revert to 3.7.0 and re-add the

<div class="btn-wrapper" <?php echo $displayData['id']; ?>>
	<?php echo $displayData['action']; ?>
</div>

The toolbar needs some improvements, which I'm no doubt going to have to do myself at some point and I'll also address this issue you've mentioned.

Could you create a new issue for that test this PR for what it does :) Let me know if there are any other errors and I'll fix them, as I've got the language to install properly now

As I already said, this PR is fine for me for what it does. Merged.