Prevent simultaneous editing of comments and issues (#31053)

fixes #22907 

Tested:
- [x] issue content edit
- [x] issue content change tasklist
- [x] pull request content edit
- [x] pull request change tasklist

![issue-content-edit](a0828889-fb96-4bc4-8600-da92e3205812)

web_src/js/features/repo-issue-edit.js

@@ -3,6 +3,7 @@ import {handleReply} from './repo-issue.js';
 import {getComboMarkdownEditor, initComboMarkdownEditor} from './comp/ComboMarkdownEditor.js';
 import {createDropzone} from './dropzone.js';
 import {GET, POST} from '../modules/fetch.js';
+import {showErrorToast} from '../modules/toast.js';
 import {hideElem, showElem} from '../utils/dom.js';
 import {attachRefIssueContextPopup} from './contextpopup.js';
 import {initCommentContent, initMarkupContent} from '../markup/content.js';
@@ -124,11 +125,17 @@ async function onEditContent(event) {
       const params = new URLSearchParams({
         content: comboMarkdownEditor.value(),
         context: editContentZone.getAttribute('data-context'),
+        content_version: editContentZone.getAttribute('data-content-version'),
       });
       for (const fileInput of dropzoneInst?.element.querySelectorAll('.files [name=files]')) params.append('files[]', fileInput.value);
 
       const response = await POST(editContentZone.getAttribute('data-update-url'), {data: params});
       const data = await response.json();
+      if (response.status === 400) {
+        showErrorToast(data.errorMessage);
+        return;
+      }
+      editContentZone.setAttribute('data-content-version', data.contentVersion);
       if (!data.content) {
         renderContent.innerHTML = document.getElementById('no-content').innerHTML;
         rawContent.textContent = '';