Bug 1188547 - Small screen view stretches media prompt background [r=andreio]

browser/components/loop/content/shared/css/conversation.css

@@ -1380,7 +1380,9 @@ html[dir="rtl"] .standalone .room-conversation-wrapper .room-inner-info-area {
   background-repeat: no-repeat;
   border: 1rem #fff solid;
   box-shadow: 0 0 5px #000;
-  margin: 0;
+  margin: auto;
+  /* `width` here is specified by the design spec. */
+  width: 250px;
 }
 
 .standalone .prompt-media-message.chrome {