In the Profile for front-end editing, the Editor width is set to 100%.

But still, in the front-end the editor takes exactly the width of the buttons I have in the editor (which is not very much since there are not many buttons 😃 ).
I also allowed resizing... but it allows to make it less wide or higher... but not wider.

Notes:
1. the size of the editor stays the same even if I change the width to 1000 px for instance.
2. All the xtd-buttons are hidden to that type of users. But if I login as Super User, the xtd-buttons are displayed... and the JCE editor becomes wider (but still not 100%)

What have I missed ?
Txs !

Yep, looking at the code in the console I saw :

<div class="editor wf-editor-container mb-2" style="max-width: 768px;">

Even with Protostar I have the same issue.
Besides I use EasyLayouts on that page, but I would not think it interferes.
Anyway, by adding this css I have a workaround :

.editor.wf-editor-container {
max-width: 100% !important;
}

Besides, on all pages (even pages without EasyLayout, pure regular Joomla pages), I noticed that the "resize editor" icon would be like "left-aligned".
Even with protostar
So I had to add this css as a workaround :

/* JCE - the resize icon at the bottom of the Editor is left aligned for some reason. This is a fix */
div#jform_articletext_resize, div#akengage-comment-editor_resize {
width: 100%;
}
span.mceIcon.mce_resize {
width: 100% !important;
}

Do you know why I have these two side-effects even with Protostar?
(the site was built originally by someone else, so it could be that there is a parameter somewhere that introduces those "funny" things)

Txs for the quick reaction Ryan.

Yes indeed I can see that it is all OK on your demo.

Let's leave it like that: I have found a workaround with my custom.css.

But there is something at hand. Is it JCE generating this line of HTML for example ?
<div class="editor wf-editor-container mb-2" style="max-width: 768px;">