Mobile: Topic list and topic (#19)

scss/topic.scss

@@ -47,6 +47,14 @@
 }
 
 #topic-title {
+  @include breakpoint(extra-large, $rule: min-width) {
+    z-index: 999;
+    padding: var(--spacing-block-l) var(--spacing-inline-xl);
+    position: sticky;
+    margin-bottom: 0;
+    top: var(--header-offset);
+    background: var(--d-content-background);
+  }
   z-index: z("composer", "content") - 1;
   padding: var(--spacing-block-l) var(--spacing-inline-xl);
   position: sticky;
@@ -55,7 +63,6 @@
   background: var(--d-content-background);
   border-radius: var(--d-border-radius-large);
 }
-
 .container.posts,
 .more-topics__container,
 #topic-footer-buttons {