don't maintain position whenever there are no parents (#4277)

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

···
           onEndReachedThreshold={2}
           onScrollToTop={onScrollToTop}
           maintainVisibleContentPosition={
-            isNative ? MAINTAIN_VISIBLE_CONTENT_POSITION : undefined
+            isNative && hasParents
+              ? MAINTAIN_VISIBLE_CONTENT_POSITION
+              : undefined
           }
           // @ts-ignore our .web version only -prf
           desktopFixedHeight