Pin action buttons to the viewport while a wide table scrolls

On a wide index table the whole page scrolls horizontally, which carried the
action items (New X, filters, etc.) off to the right with it. Make
#titlebar_right position:sticky so the buttons stay pinned to the right of the
viewport while the table scrolls underneath — matching the sticky header.

Consuming apps each shipped this as a local scroll_buttons_init.scss; it lives
in the theme now so every app behaves the same.

Author: Fivell

app/assets/stylesheets/wigu/active_admin_theme.scss

@@ -672,6 +672,13 @@ body.active_admin {
       }
     }
     #titlebar_right {
+      // Sticky action buttons: on a wide index table the whole page scrolls
+      // horizontally, so pin the action items to the right of the viewport —
+      // the page scrolls, the buttons stay put. Each app used to carry this as a
+      // local scroll_buttons_init.scss; it lives in the theme now.
+      position: sticky;
+      right: 15px;
+      float: right;
       // `> a` and `> .dropdown_menu > a`: an action item may hold a dropdown, and
       // a descendant selector here paints its menu entries with the button fill
       // too — with !important, so the entry rules below cannot undo it.