Bug 1868796 - Fix chat acount settings scrolling. r=aleca

Differential Revision: https://phabricator.services.mozilla.com/D195786

--HG--
extra : rebase_source : eabec3506a771c592aac0d145cb458881c2c7849

mail/components/im/content/am-im.xhtml

@@ -39,7 +39,7 @@
   <html:body
     xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
   >
-    <vbox flex="1" style="overflow: auto; padding: 0"
+    <vbox flex="1" style="overflow: auto; padding: 0; max-height: 100vh"
       ><vbox id="containerBox" flex="1">
         <hbox class="dialogheader">
           <label