inbox: Attempt to fix inbox scrolling to top unexpectedly.

Inbox can suddenly scroll to top due to a rerender when user
is trying to scroll down. A possible caues for this is
`move_focus_to_visible_area` being triggered on scroll but the
element located using:

```
document.elementFromPoint(inbox_center_x, inbox_row_below_filters);
```

being out of view. To fix it, we try to locate the element in the
same animation frame. This will remove the possibility of element
being out of view when we try to set focus to it.

web/src/inbox_ui.ts

@@ -2123,8 +2123,14 @@ export function initialize({hide_other_views}: {hide_other_views: () => void}):
     $(document).on(
         "scroll",
         _.throttle(() => {
-            move_focus_to_visible_area();
-        }, 50),
+            if (!inbox_util.is_visible()) {
+                // This check is duplicated with move_focus_to_visible_area. It
+                // is worth doing to avoid the performance hit of wrapping
+                // requestAnimationFramearound a likely noop.
+                return;
+            }
+            requestAnimationFrame(move_focus_to_visible_area);
+        }, 100),
     );
 
     $("body").on(