Fix iframe unwanted horizontal scrollbar.

conversejs/builtin.ts

@@ -97,6 +97,8 @@ window.initConverse = async function initConverse ({
     const body = document.querySelector('body')
     if (body) {
       body.classList.add('livechat-iframe')
+      // prevent horizontal scrollbar when in iframe. (don't know why, but does not work if done by CSS)
+      body.style.overflowX = 'hidden'
     }
   }