Add bottom border to messages. Fixes #8836 (#8875)

Co-authored-by: Thibaud Colas <thibaudcolas@gmail.com>

CHANGELOG.txt

@@ -76,6 +76,7 @@ Changelog
  * Add an extra confirmation prompt when deleting pages with a large number of child pages (Jaspreet Singh)
  * Adopt the slim header in page listing views, with buttons moved under the "Actions" dropdown (Paarth Agarwal)
  * Improve help block styles in Windows High Contrast Mode with less reliance on communication via colour alone (Anuja Verma)
+ * Add a bottom border to top messages so they stand out from the header (Anuja Verma)
  * Replace latin abbreviations (i.e. / e.g.) with common English phrases so that documentation is easier to understand (Dominik Lech)
  * Add shortcut for accessing StreamField blocks by block name with new `blocks_by_name` and `first_block_by_name` methods on `StreamValue` (Tidiane Dia, Matt Westcott)
  * Extend support for custom user interface colours across almost all admin colours (Thibaud Colas)

client/scss/components/_messages.scss

@@ -20,6 +20,7 @@
     // @include nice-padding;
     padding: 1.6em 3em 1.6em 1.6em;
     color: $color-white;
+    border-bottom: 1px solid transparent;
   }
 
   > ul > li:before {

docs/releases/4.0.md

@@ -100,6 +100,7 @@ In Wagtail 2.12, we introduced theming support for Wagtail’s primary brand col
  * Add an extra confirmation prompt when deleting pages with a large number of child pages (Jaspreet Singh)
  * Adopt the slim header in page listing views, with buttons moved under the "Actions" dropdown (Paarth Agarwal)
  * Improve help block styles in Windows High Contrast Mode with less reliance on communication via colour alone (Anuja Verma)
+ * Add a bottom border to top messages so they stand out from the header (Anuja Verma)
  * Replace latin abbreviations (i.e. / e.g.) with common English phrases so that documentation is easier to understand (Dominik Lech)
  * Add shortcut for accessing StreamField blocks by block name with new [`blocks_by_name` and `first_block_by_name` methods on `StreamValue`](streamfield_retrieving_blocks_by_name) (Tidiane Dia, Matt Westcott)
  * Add HTML-aware max_length validation on RichTextField and RichTextBlock (Matt Westcott)