UX: Fix chat separator alignment (#20669)

Also: work around 1px svg shift in scroll-to-bottom button

plugins/chat/assets/stylesheets/common/base-common.scss

@@ -384,6 +384,7 @@ $float-height: 530px;
 
       .d-icon {
         color: var(--secondary);
+        margin-left: 1px; // "fixes" the 1px svg shift
       }
     }
 

plugins/chat/assets/stylesheets/common/chat-message-separator.scss

@@ -56,7 +56,9 @@
     }
 
     .chat-message-separator__text-container {
-      padding-top: 7px;
+      align-items: center;
+      display: flex;
+      height: 40px;
       position: sticky;
       top: -1px;
 
@@ -98,11 +100,7 @@
 
       .chat-message-separator__line {
         border-top: 1px solid var(--secondary-high);
-        left: 0;
         margin: 0 0 -1px;
-        position: relative;
-        right: 0;
-        top: -1px;
       }
     }
   }

plugins/chat/lib/chat/notifier.rb

@@ -130,11 +130,9 @@ module Chat
         already_covered_ids = []
 
         expand_direct_mentions(to_notify, already_covered_ids, skip_notifications)
-        if !skip_notifications
-          expand_group_mentions(to_notify, already_covered_ids)
-          expand_here_mention(to_notify, already_covered_ids)
-          expand_global_mention(to_notify, already_covered_ids)
-        end
+        expand_group_mentions(to_notify, already_covered_ids, skip_notifications)
+        expand_here_mention(to_notify, already_covered_ids, skip_notifications)
+        expand_global_mention(to_notify, already_covered_ids, skip_notifications)
 
         filter_users_ignoring_or_muting_creator(to_notify, already_covered_ids)
 
@@ -142,10 +140,10 @@ module Chat
       end
     end
 
-    def expand_global_mention(to_notify, already_covered_ids)
+    def expand_global_mention(to_notify, already_covered_ids, skip)
       has_all_mention = @mentions.has_global_mention
 
-      if has_all_mention && @chat_channel.allow_channel_wide_mentions
+      if has_all_mention && @chat_channel.allow_channel_wide_mentions && !skip
         to_notify[:global_mentions] = @mentions
           .global_mentions
           .not_suspended
@@ -159,10 +157,10 @@ module Chat
       end
     end
 
-    def expand_here_mention(to_notify, already_covered_ids)
+    def expand_here_mention(to_notify, already_covered_ids, skip)
       has_here_mention = @mentions.has_here_mention
 
-      if has_here_mention && @chat_channel.allow_channel_wide_mentions
+      if has_here_mention && @chat_channel.allow_channel_wide_mentions && !skip
         to_notify[:here_mentions] = @mentions
           .here_mentions
           .not_suspended
@@ -214,8 +212,8 @@ module Chat
       already_covered_ids.concat(to_notify[:direct_mentions])
     end
 
-    def expand_group_mentions(to_notify, already_covered_ids)
-      return if @mentions.visible_groups.empty?
+    def expand_group_mentions(to_notify, already_covered_ids, skip)
+      return [] if skip || @mentions.visible_groups.empty?
 
       reached_by_group =
         @mentions