Ensure that toasts are always visible, even when scrolling

This is a hacky workaround, but a proper fix would require quite some refactoring. Considering that this hack is pretty isolated and not going to affect any other parts of the UI and that we will need to upgrade to Blueprint 5 at some point anyway, I’ve opted for the quick-and-dirty solution for now.

ui/src/app/App.scss

@@ -141,8 +141,11 @@ a {
   );
 }
 
-/* This causes toasts to appear *below* the navbar instead of on top of it */
 .aleph-toaster {
+  /* Hacky workaround to ensure toasts are visible even when scrolling */
+  position: fixed !important;
+
+  /* This causes toasts to appear *below* the navbar instead of on top of it */
   margin-top: $aleph-grid-size * 5;
 }