Try and handle conflict on update: it's a plan

app/Entities/Controllers/PageController.php

@@ -222,6 +222,23 @@ public function update(Request $request, string $bookSlug, string $pageSlug)
         $page = $this->queries->findVisibleBySlugsOrFail($bookSlug, $pageSlug);
         $this->checkOwnablePermission('page-update', $page);
 
+        $editorRevision = $request->get("revision");
+        // TODO This query is repeated at app/Entities/Tools/PageEditorData.php:73
+        //      Should probably be extracted, also we could fetch other information like the user
+        //      that created the newer revision and when it happened
+        $currentRevisionId = $page->currentRevision()?->get(['id'])->first()->id;
+
+        if ($editorRevision != $currentRevisionId) {
+            // TODO This is probably all kinds of wrong
+            //      We need to send the user back to their copy of the page so they
+            //      can save it or restart from the current revision
+            $this->showWarningNotification(trans("errors.page_update_conflict", [
+                "editorRevision" => $editorRevision,
+                "currentRevision" => $currentRevisionId
+            ]));
+            return $this->edit($request, $bookSlug, $pageSlug);
+        }
+
         $this->pageRepo->update($page, $request->all());
 
         return redirect($page->getUrl());

app/Entities/Tools/PageEditorData.php

@@ -70,6 +70,7 @@ protected function build(): array
             'draftsEnabled'   => $draftsEnabled,
             'templates'       => $templates,
             'editor'          => $editorType,
+            'revision'        => $page->currentRevision()->get(['id'])?->first()?->id,
             'comments'        => new CommentTree($page),
         ];
     }

lang/en/errors.php

@@ -64,6 +64,7 @@
     'page_draft_autosave_fail' => 'Failed to save draft. Ensure you have internet connection before saving this page',
     'page_draft_delete_fail' => 'Failed to delete page draft and fetch current page saved content',
     'page_custom_home_deletion' => 'Cannot delete a page while it is set as a homepage',
+    'page_update_conflict' => 'The page was edited between your revision (:editorRevision) and now (:currentRevision)',
 
     // Entities
     'entity_not_found' => 'Entity not found',

resources/views/pages/parts/form.blade.php

@@ -33,6 +33,7 @@
                 {{--Editors--}}
                 <div class="edit-area flex-fill flex">
                     <input type="hidden" name="editor" value="{{ $editor->value }}">
+                    <input type="hidden" name="revision" value="{{ $revision }}">
 
                     @if($editor === \BookStack\Entities\Tools\PageEditorType::WysiwygLexical)
                         @include('pages.parts.wysiwyg-editor', ['model' => $model])