fix scrollDirection when direction is RTL

[DevinFrenze]

To determine the scrollDirection in the _onScrollHorizontal method it compares the previous state's scrollOffset with the current scrollLeft value of the target. For browsers whose "RTLOffsetType" is "negative", this results in the scrollDirection being set to "backward".

The scrollOffset state value has been converted to a positive number or zero, and the scrollLeft value of the target (when "RTLOffsetType" is "negative") is a negative number. When comparing prevState.scrollOffset < scrollLeft it is always false because a positive value or zero is never greater than a negative value.

Proposed fix: compare prevState.scrollOffset < scrollOffset because scrollOffset has already been transformed in the same way that prevState.scrollOffset was transformed.

This pull request is intended to address this bug #689.

[DevinFrenze]

@bvaughn sorry to ping you, but if you have a moment to glance at this and let me know if you think it's a reasonable change i would appreciate it. i'm using react-window in a project and would much rather get this change in and use the main repo than be using a fork indefinitely.

and for what it's worth i have tested this some amount and believe that it works as expected.

[DevinFrenze]

@bvaughn i'm requesting your review here. let me know if there's someone else i should reach out to for review.

[bvaughn]

I think this change looks reasonable. I haven't worked in this code in many months though so I'm not totally confident in my own assessment. Have you tested it?

[DevinFrenze]

I tested it in a project that I'm developing and it works well there, and it doesn't change behavior when using react-window left-to-right. Of course, there may be edge cases I don't know about.

[bvaughn]

That's fair. Okay!

[bvaughn]

@DevinFrenze Please check 1.8.10 (just published) and confirm the fix?

[DevinFrenze]

@bvaughn it appears to be working well ! thanks for merging

[bvaughn]

No problem. Thanks for verifying!