Expanding fr (#2221)

* forced fr to expand

* margin size

* remove comment

* missing snapshot

* snapshot tests

* changelog

* optimize

* snapshot fix

* snapshot update

* snapshot and fixes

* docstrings [skip ci]

docs/examples/widgets/content_switcher.css

@@ -1,9 +1,9 @@
 Screen {
     align: center middle;
+    padding: 1;
 }
 
 #buttons {
-    margin-top: 1;
     height: 3;
     width: auto;
 }
@@ -12,7 +12,7 @@ ContentSwitcher {
     background: $panel;
     border: round $primary;
     width: 90%;
-    height: 80%;
+    height: 1fr;
 }
 
 DataTable {