Set max for Zen mode textarea to screen height

app/assets/stylesheets/framework/zen.scss

@@ -20,8 +20,9 @@
       outline: none;
       resize: none;
       height: 100vh;
+      max-height: calc(100vh - 10px);
       max-width: 900px;
-      margin: 0 auto;
+      margin: 0 auto 10px;
     }
 
     .zen-control-leave {

changelogs/unreleased/28704-fullscreen-zen-mode-is-broken.yml

+---
+title: Set max height to screen height for Zen mode
+merge_request: 9667
+author: