Add test for sidebar display value on mobile

Author: GuillaumeGomez

src/test/rustdoc-gui/sidebar-mobile.goml

@@ -0,0 +1,10 @@
+// This test ensure that the sidebar isn't "hidden" on mobile but instead moved out of the viewport.
+// This is especially important for devices for "text-first" content (like for users with
+// sight issues).
+goto: file://|DOC_PATH|/test_docs/struct.Foo.html
+// Switching to "mobile view" by reducing the width to 600px.
+size: (600, 600)
+assert-css: (".sidebar-elems", {"display": "block", "left": "-246px"})
+// Opening the sidebar menu.
+click: ".sidebar-menu"
+assert-css: (".sidebar-elems", {"display": "block", "left": "0px"})