compose: Ensure textarea is hidden in compose preview.

This corrects a subtle bug where being zoomed in in
a narrow window causes textarea bleedthrough behind
elements like the audio player.

web/styles/compose.css

@@ -2027,14 +2027,19 @@ textarea.new_message_textarea {
         }
     }
 
+    .message-textarea,
     .message-edit-textbox {
         /* In preview mode, we hide the message edit
            textbox to prevent it from appearing behind
            the preview container. We cannot set its
            display to none or visibility to hidden, as
            that would prevent us from modifying the
-           textarea's value. */
-        height: 0;
-        overflow: hidden;
+           textarea's value.
+
+           The !important use here in necessary to be sure
+           we override the inline styles set on the
+           compose textarea. */
+        height: 0 !important;
+        overflow: hidden !important;
     }
 }