[Better Expense Reports] Fix FlatList two-way scroll locally

[Guccio163]

Explanation of Change

PR ensures that web implementation of inverted FlatList scrolls in only one direction at a time by comparing X and Y deltas, then choosing the direction with the bigger delta. A follow-up as per this comment to find a fix locally, not to maintain the patch forever - it isn't a bug to be fixed upstream.

Fixed Issues

$ #60276 (comment)
PROPOSAL:

Tests

1. Go to the chat/workspace
2. Create as many transactions as it's necessary to be able to scroll Money Request Report
3. Check if scrolling Money Request Report horizontally with touchpad/mouse wheel works
4. Ensure that it is impossible to scroll diagonally/scroll Money Request Report horizontally and all reports vertically at the same time. Diagonal swipe on the touchpad should scroll only horizontally OR vertically, depends which delta is bigger.

• Verify that no errors appear in the JS console

Offline tests

QA Steps

Same as tests

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
  • MacOS: Desktop
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
  • I verified that any callback methods that were added or modified are named for what the method does and never what callback they handle (i.e. toggleReport and not onIconClick)
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  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified any copy / text shown in the product is localized by adding it to src/languages/* files and using the translation method
    • If any non-english text was added/modified, I used JaimeGPT to get English > Spanish translation. I then posted it in #expensify-open-source and it was approved by an internal Expensify engineer. Link to Slack message:
  • I verified all numbers, amounts, dates and phone numbers shown in the product are using the localization methods
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• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
• I verified all code is DRY (the PR doesn't include any logic written more than once, with the exception of tests)
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  • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
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  • A similar style doesn't already exist
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• If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
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  • I added Design label and/or tagged @Expensify/design so the design team can review the changes.
• If a new page is added, I verified it's using the ScrollView component to make it scrollable when more elements are added to the page.
• I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

web.mov

MacOS: Desktop

desktop.mov

[github-actions]

⚠️ This PR is possibly changing native code and/or updating libraries, it may cause problems with HybridApp. Please check if any patch updates are required in the HybridApp repo and run an AdHoc build to verify that HybridApp will not break. Ask Contributor Plus for help if you are not sure how to handle this. ⚠️

[melvin-bot]

@DylanDylann Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

[trjExpensify]

@DylanDylann are you on track to review this?

[DylanDylann]

I will try to complete my review today. Or at least, I will complete it on Monday for sure

[DylanDylann]

@Guccio163 Did you test this change on the Hybrid App? It caused the crash App because addEventListener is only available on web platform

[Guccio163]

You're right, I didn't - I'll try to comment on Monday; Since addEventListener is only available on web (and hence desktop) the only 2 solutions I see right now are:

1. wrapping code in platform === CONST.PLATFORM.WEB || platform === CONST.PLATFORM.DESKTOP
2. leaving the solution in patch

Since we can't really push the patch upstream (it's not really a bug) and FlatList doesn't support a onWheel function to handle events just like we need to, it doesn't leave us a lot of space to work - I'll consult with the SWM and get back to you 🔜

[Guccio163]

Finally, I solved it platform agnostic, just checking whether addEventListener is available - if it is then it is web/desktop environment and should be applied; if not - it won't bother. Works on every platform so ready for a second look, LMKWDYT @DylanDylann 👀

[DylanDylann]

I tested and noticed that the bug isn't fixed completely

Screen.Recording.2025-05-06.at.16.57.19.mov

[DylanDylann]

Suggested change

	const node = carouselRef.current?.getScrollableNode?.() as HTMLElement;
	const node = carouselRef.current?.getScrollableNode?.() as HTMLElement;
	if (!(node instanceof HTMLElement)) {
	return;
	}

[Guccio163]

I tested and noticed that the bug isn't fixed completely

Right, because of mouse/wheel events' characteristics this is most of what we can do with a local change like that without big interference into making wrappers, editing ReportActions list etc. The one idea still available is implementing FlatList in GestureHandler - I'll get back to you later this week with some updates!

[DylanDylann]

@mountiny @Guccio163 I think we should keep the patch file to solve this bug completely with a smooth transition. Wdyt?

[Guccio163]

GestureHandler tested so time for a summary:

Theoretically, the easiest and simplest solution would be to implement an algorithm like "only deltaX difference when scrolling Carousel, only deltaY when scrolling whole list", but there is no way to deterministically detect that on web - there are no wheelStart/wheelEnd events, just a plain 'wheelEvent' and/or 'mouseMove'.

The GestureHandler handles the wheel event with some start/end detections, but due to the difficulties stated above it isn't perfect as well and have to rely on some sort of heuristics (isn't quite safe/reliable to use in this case):

pan.mov

The way to go in our scenario is stating constant rules for event handling, just like it is now done in patch and how I aim to finish it here - with a constant eventListener. The problem is that a single swap can generate many wheelEvents and they vary in parameters - that sometimes leads to a situation where among many horizontal wheels, occur single micro events with very small deltaY but even smaller deltaX and that's why it moves vertically a little.

Interesting possible solution in using Timeout (f.ex. if user doesn't wheel for longer than 0.2 seconds then assume that he stopped scrolling) and boolean conditional stating whether we currently block vertical or horizontal events; It isn't perfect but should do the job.

BTW: By twitching Math.abs(deltaX) >= Math.abs(deltaY) we can ensure a little wider window of error for horizontal scrolls, f.ex. 3* Math.abs(deltaX) >= Math.abs(deltaY)

Not an ideal position - I'd probably stay with the current patch or eventListener from this PR, but I'm curious WDYT @mountiny @DylanDylann ?

[mountiny]

Thanks for the investigation, i agree that if the parch solution is the best for now, lets keep it and focus on other things. We could re-visit this later if we still see some issues with the behaviour and would like to clear the patch

[DylanDylann]

Agree 👍

[Guccio163]

Works for me, summary is gathered here for the next attempt 👍

[DylanDylann]

I think we can close this PR

[trjExpensify]

Closing!