Quick quick correction, it seems a textarea containing html code that includes style elements is displayed as an empty textarea!

Replaced with a classic text input, tinymce picks it up correctly...

app/assets/javascripts/tinymce.coffee.erb

@@ -2,9 +2,10 @@ $(document).on 'turbolinks:load', ->
   # Setup the graphical editor for event's description
   tinyMCE.init
     schema: 'html5',
+    height: '40em',
     branding: false,
     language: 'fr_FR',
-    selector: 'textarea.description',
+    selector: 'input.description',
     content_css: '<%= asset_path 'application.css' %>',
     entity_encoding : 'raw',
     add_unload_trigger: true,

app/views/events/_form.html.haml

@@ -41,7 +41,8 @@
       :markdown
         #{t '.description_helper'}
     = f.label :description
-    = f.text_area :description, rows: 25, class: :description
+    /= f.text_area :description, rows: 25, class: :description
+    = f.text_field :description, class: :description
 
   .field.place_name
     = f.label :place_name

app/views/orgas/_form.html.haml

@@ -20,7 +20,8 @@
     = f.text_field :name, required: true
   .field.description
     = f.label :description
-    = f.text_area :description, rows: 25, class: :description
+    /= f.text_area :description, rows: 25, class: :description
+    = f.text_field :description, class: :description
   .field.place_name
     = f.label :place_name
     = f.text_field :place_name

public/assets/tinymce-8da8cfeaca8ef6e55283f3851197317fcf3acfeebb4642f0a0a34fcf30e8d7d0.js

@@ -0,0 +1,22 @@
+(function() {
+  $(document).on('turbolinks:load', function() {
+    return tinyMCE.init({
+      schema: 'html5',
+      height: '40em',
+      branding: false,
+      language: 'fr_FR',
+      selector: 'input.description',
+      content_css: '/assets/application-31446b61376e114e9c8dd48e3beb3218fd819e0de7fb8bb309580ce7e688bb5d.css',
+      entity_encoding: 'raw',
+      add_unload_trigger: true,
+      browser_spellcheck: true,
+      toolbar: [' cut copy paste | undo redo | searchreplace | link image media charmap table | code visualblocks preview fullscreen', ' removeformat | bold italic strikethrough | superscript subscript | bullist numlist | alignleft aligncenter alignright alignjustify alignnone | outdent indent'],
+      plugins: 'lists, advlist, autolink, link, image, charmap, paste, print, preview, table, fullscreen, searchreplace, media, insertdatetime, visualblocks, wordcount, contextmenu, code'
+    });
+  });
+
+  $(document).on('turbolinks:before-cache', function() {
+    return tinymce.remove();
+  });
+
+}).call(this);