Confirm Issue, test on nightly build.

We had this reported before but it was in the template editor. Look for the PR that fixed it there and apply here

@brianteeman pull request #30241?

Yes thats the one

Looks like indeed this solves the issue

diff --git a/build/media_source/plg_editors_codemirror/js/joomla-editor-codemirror.w-c.es6.js b/build/media_source/plg_editors_codemirror/js/joomla-editor-codemirror.w-c.es6.js
index ecef691..e99ae93 100644
--- a/build/media_source/plg_editors_codemirror/js/joomla-editor-codemirror.w-c.es6.js
+++ b/build/media_source/plg_editors_codemirror/js/joomla-editor-codemirror.w-c.es6.js
@@ -42,4 +42,10 @@
     const that = this;
 
+    // Check if instance exists to avoid duplication on resize
+    if (this.instance !== '') {
+      Joomla.editors.instances[this.element.id] = this.instance;
+       return;
+    }
+
     // For mode autoloading.
     window.CodeMirror.modeURL = this.getAttribute('mod-path');

That's not the correct solution since the problem is not in CodeMirror, a similar issue exists with tinyMCE #33101
The problem is that tabs are actually moving DOM elements when switching from tabs to accordion therefore the proper solution would be to fix the tabs to not move the DOM elements.

a similar issue exists with tinyMCE

We had this reported before but it was in the template editor.

The problem with the template editor, when using out of the box configuration (CodeMirror) still remains today unresolved.

using out of the box configuration

What do you mean here? Can you provide an example?

I thought it was fixed as well but I can confirm its broken

Simple.

1. Install Joomla 4 from 4.0-dev today
2. Navigate to System -> Site Templates -> Cassiopeia Details and Files -> Click Index.php
3. Resize the screen:

Yeah, that's from tabs and the stupid mode change to accordion. I shouldn't have pushed that stupid code...

Its been like this for quite some time... I reported it in 2018... #22778

So... What now?

Please test PR #34813