Move to correct selector

Also delete a -webkit prefix that is not needed

app/assets/stylesheets/framework/flash.scss

@@ -9,9 +9,15 @@ $notification-box-shadow-color: rgba(0, 0, 0, 0.25);
 
   &.sticky {
     position: sticky;
-    position: -webkit-sticky;
     top: $flash-container-top;
     z-index: 251;
+
+    .flash-alert,
+    .flash-notice,
+    .flash-success,
+    .flash-warning {
+      @include gl-mb-4;
+    }
   }
 
   &.flash-container-page {

app/assets/stylesheets/pages/merge_requests.scss

@@ -1039,9 +1039,3 @@ $mr-widget-min-height: 69px;
 .diff-file-row.is-active {
   background-color: $gray-50;
 }
-
-.merge-request-container {
-  .flash-container {
-    @include gl-mb-4;
-  }
-}

changelogs/unreleased/psi-mr-space.yml

+---
+title: Reduce whitespace on MR page header
+merge_request: 45966
+author:
+type: fixed