Fix dropdown going off top of screen

The previous solution didn't properly account for document scroll, so when replying to posts, the parent offset would be extremely large, and it'd fall back to the top coordinate, which is out of bounds on small screens.

js/src/forum/addComposerAutocomplete.js

@@ -172,7 +172,7 @@ export default function addComposerAutocomplete() {
               }
 
               // Prevent the dropdown from going off screen on mobile
-              top = Math.max(-parent.offset().top, top);
+              top = Math.max(-(parent.offset().top - $(document).scrollTop()), top);
               left = Math.max(-parent.offset().left, left);
 
               dropdown.show(left, top);