Expose scrollOffset for testability

[badeball]

Hi,

Due to the asynchronous nature of rendering in React, there's a window in time where one might have mutated an element's scrollLeft property and where the page still appear to be rendering elements based on the previous value. This is insignificantly small to affect the user experience, but it does affect attempts to automate interaction.

We have an unsorted, virtualized table, which we test. Our tests mimics user behavior and "searches" the table by resetting scrollTop and scrolling iteratively until the target can be found. However, when resetting scrollTop or increasing it, we have to wait a fixed amount of time to "be sure" that the table has in fact re-rendered, otherwise we risk re-querying the same values and scrolling past the target. The browser can be quite busy in our case, so this becomes significant.

By exposing scrollOffset and rendering it as part of the DOM (think data-scroll-offset={scrollOfset}), we can mutate scrollTop and simply wait for the attribute to change. This time is almost always insignificant and the approach much more preferable than adding default, static waits to every test and slowing the entire suite.

We solved this in 2.4.10 with patch-package and the following patch, and this works well.

diff --git a/node_modules/react-virtual/dist/react-virtual.mjs b/node_modules/react-virtual/dist/react-virtual.mjs
index ef9e63c..cfcf772 100644
--- a/node_modules/react-virtual/dist/react-virtual.mjs
+++ b/node_modules/react-virtual/dist/react-virtual.mjs
@@ -440,6 +440,7 @@ function useVirtual(_ref) {
     });
   }, [tryScrollToIndex]);
   return {
+    scrollOffset: scrollOffset,
     virtualItems: virtualItems,
     totalSize: totalSize,
     scrollToOffset: scrollToOffset,
diff --git a/node_modules/react-virtual/types/index.d.ts b/node_modules/react-virtual/types/index.d.ts
index e475a1b..98f2952 100644
--- a/node_modules/react-virtual/types/index.d.ts
+++ b/node_modules/react-virtual/types/index.d.ts
@@ -53,6 +53,7 @@ export interface Options<T> {
 }
 
 declare function useVirtual<T>(options: Options<T>): {
+  scrollOffset: number
   virtualItems: VirtualItem[]
   totalSize: number
   scrollToOffset: (index: number, options?: ScrollToOffsetOptions) => void

My question is, can you expose this state?