[Web] Fix thread jumps (#9111)

* [Web] Fix thread jumps

* Comment formatting

---------

Co-authored-by: Eric Bailey <git@esb.lol>

src/screens/PostThread/index.tsx

@@ -148,7 +148,9 @@ export function PostThread({uri}: {uri: string}) {
    */
   const shouldHandleScroll = useRef(true)
   /**
-   * Called any time the content size of the list changes, _just_ before paint.
+   * Called any time the content size of the list changes. Could be a fresh
+   * render, items being added to the list, or any resize that changes the
+   * scrollable size of the content.
    *
    * We want this to fire every time we change params (which will reset
    * `deferParents` via `onLayout` on the anchor post, due to the key change),
@@ -193,24 +195,23 @@ export function PostThread({uri}: {uri: string}) {
        * will give us a _positive_ offset, which will scroll the anchor post
        * back _up_ to the top of the screen.
        */
-      list.scrollToOffset({
-        offset: anchorOffsetTop - headerHeight,
-      })
+      const offset = anchorOffsetTop - headerHeight
+      list.scrollToOffset({offset})
 
       /*
-       * After the second pass, `deferParents` will be `false`, and we need
-       * to ensure this doesn't run again until scroll handling is requested
-       * again via `shouldHandleScroll.current === true` and a params
-       * change via `prepareForParamsUpdate`.
+       * After we manage to do a positive adjustment, we need to ensure this
+       * doesn't run again until scroll handling is requested again via
+       * `shouldHandleScroll.current === true` and a params change via
+       * `prepareForParamsUpdate`.
        *
        * The `isRoot` here is needed because if we're looking at the anchor
        * post, this handler will not fire after `deferParents` is set to
        * `false`, since there are no parents to render above it. In this case,
-       * we want to make sure `shouldHandleScroll` is set to `false` so that
-       * subsequent size changes unrelated to a params change (like pagination)
-       * do not affect scroll.
+       * we want to make sure `shouldHandleScroll` is set to `false` right away
+       * so that subsequent size changes unrelated to a params change (like
+       * pagination) do not affect scroll.
        */
-      if (!deferParents || isRoot) shouldHandleScroll.current = false
+      if (offset > 0 || isRoot) shouldHandleScroll.current = false
     }
   })