rustdoc ui: adjust tooltip z-index to be above sidebar

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

@@ -1046,7 +1046,7 @@ so that we can apply CSS-filters to change the arrow color in themes */
 	position: absolute;
 	top: 100%;
 	right: 0;
-	z-index: 2;
+	z-index: 101;
 	margin-top: 7px;
 	border-radius: 3px;
 	border: 1px solid var(--border-color);

tests/rustdoc-gui/tooltip-over-sidebar.goml

@@ -0,0 +1,12 @@
+// Check that the doctest info tooltips are above the sidebar.
+go-to: "file://" + |DOC_PATH| + "/test_docs/fn.foo.html"
+move-cursor-to: ".example-wrap.ignore .tooltip"
+wait-for: ".tooltip.popover"
+
+// Move cursor to top left corner of the tooltip and check that it doesn't fade.
+move-cursor-to: ".tooltip.popover"
+wait-for: 100
+assert: ".tooltip.popover:not(.fade-out)"
+
+move-cursor-to: (0, 0)
+wait-for: ".tooltip.popover.fade-out"