Fix double scrollbar issue in SourcesTree (#5787)

Author: darkwing

src/components/PrimaryPanes/Sources.css

@@ -65,11 +65,6 @@
   overflow-y: auto;
 }
 
-.sources-list {
-  flex: 1;
-  display: flex;
-}
-
 .sources-list .tree:focus {
   outline: none;
 }
@@ -226,6 +221,11 @@
   display: block;
 }
 
+.sources-list-custom-root .sources-list {
+  flex: 1;
+  display: flex;
+}
+
 .sources-list-custom-root .sources-list,
 .sources-list-custom-root .no-sources-message {
   position: absolute;