fix(311-sidebar-toggle): Set tabindex=0 on ui output parent element

`uiOutput()` elements now get `display:contents`,
which means they can't receive tab focus. The tabindex
is used in 311 to test correct tab order, not for any real
functionality in the app.

inst/apps/311-bslib-sidebar-toggle-methods/app.R

@@ -64,7 +64,7 @@ ui <- page_fixed(
         border = FALSE,
         border_radius = FALSE,
         h2("Sidebar Layout"),
-        uiOutput("ui_content", tabindex = 0),
+        div(uiOutput("ui_content"), tabindex = 0),
       )
     )
   ),