Fix safari post field display issue in single column layout (#10883)

app/javascript/styles/mastodon/components.scss

@@ -1976,6 +1976,7 @@ a.account__display-name {
 .columns-area--mobile {
   flex-direction: column;
   width: 100%;
+  height: 100%;
   margin: 0 auto;
 
   .column,