feat(rustdoc): open sidebar menu when links inside it are focused

Fixes rust-lang#87172

Based on rust-lang#87167 (which should be merged first)

Preview it at https://notriddle.com/notriddle-rustdoc-test/std/index.html

Co-authored-by: Guillaume Gomez <guillaume.gomez@huawei.com>

src/librustdoc/html/static/css/rustdoc.css

@@ -1729,7 +1729,8 @@ details.undocumented[open] > summary::before {
 		width: calc(100% + 30px);
 	}
 
-	.show-it {
+	.show-it, .sidebar-elems:focus-within {
+		z-index:  2;
 		left: 0;
 	}
 

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

@@ -8,3 +8,13 @@ assert-css: (".sidebar-elems", {"display": "block", "left": "-246px"})
 // Opening the sidebar menu.
 click: ".sidebar-menu"
 assert-css: (".sidebar-elems", {"display": "block", "left": "0px"})
+// Closing the sidebar menu.
+click: ".sidebar-menu"
+assert-css: (".sidebar-elems", {"display": "block", "left": "-246px"})
+// Force the sidebar open by focusing a link inside it.
+// This makes it easier for keyboard users to get to it.
+focus: ".sidebar-title"
+assert-css: (".sidebar-elems", {"display": "block", "left": "0px"})
+// When we tab out of the sidebar, close it.
+focus: ".search-input"
+assert-css: (".sidebar-elems", {"display": "block", "left": "-246px"})