Present highlighted post as part of the thread (#2628)

src/view/com/post-thread/PostThreadItem.tsx

···
 
         <View
           testID={`postThreadItem-by-${post.author.handle}`}
-          style={[styles.outer, styles.outerHighlighted, pal.border, pal.view]}
+          style={[
+            styles.outer,
+            styles.outerHighlighted,
+            rootUri === post.uri && styles.outerHighlightedRoot,
+            pal.border,
+            pal.view,
+          ]}
           accessible={false}>
           <PostSandboxWarning />
           <View style={styles.layout}>
···
       paddingLeft: 8,
     },
     outerHighlighted: {
-      paddingTop: 16,
+      borderTopWidth: 0,
+      paddingTop: 4,
       paddingLeft: 8,
       paddingRight: 8,
+    },
+    outerHighlightedRoot: {
+      borderTopWidth: 1,
+      paddingTop: 16,
     },
     noTopBorder: {
       borderTopWidth: 0,