Bug 1799658 - Port bug 1795049: Keep shrinking main pane with viewport width. r=aleca

Differential Revision: https://phabricator.services.mozilla.com/D161589

--HG--
extra : amend_source : 5343b2e7fb8ac3b34630265cbbd27912004d5426

mail/themes/shared/mail/preferences/preferences.css

@@ -163,7 +163,8 @@ menulist::part(dropmarker-icon) {
 
 .paneDeckContainer {
   display: block;
-  width: 800px;
+  width: min(100%, 800px);
+  min-width: min-content;
 }
 
 .sticky-container {