[$500] mWeb - Chat - Tapping on new messages after marking a message unread leads to a blank page

[kbecciv]

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Version Number: 1.4.1.5
Reproducible in staging?: y
Reproducible in production?: y
If this was caught during regression testing, add the test name, ID and link from TestRail:
Email or phone of affected tester (no customers):
Logs: https://stackoverflow.com/c/expensify/questions/4856
Expensify/Expensify Issue URL:
Issue reported by: Applause - Internal Team
Slack conversation:

Action Performed:

1. Navigate to https://staging.new.expensify.com/
2. Log in
3. Open a group conversation with a lot of messages
4. Swipe to the middle of the page and mark any message as unread
5. Swipe to the top of the page
6. Tap on the "New messages" button

Expected Result:

User should be redirected to the unread message.

Actual Result:

Tapping on new messages after marking a message unread leads to a blank page

Workaround:

Unknown

Platforms:

Which of our officially supported platforms is this issue occurring on?

• Android: Native
• Android: mWeb Chrome
• iOS: Native
• iOS: mWeb Safari
• MacOS: Chrome / Safari
• MacOS: Desktop

Screenshots/Videos

Add any screenshot/video evidence

Bug6284625_1700519015921.Group_disappears.mp4

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• wlegolas | Contributor | 27951745

[melvin-bot]

Triggered auto assignment to @JmillsExpensify (Bug), see https://stackoverflow.com/c/expensify/questions/14418 for more details.

[melvin-bot]

Job added to Upwork: https://www.upwork.com/jobs/~017b6f62003e67ff2c

[melvin-bot]

Bug0 Triage Checklist (Main S/O)

• This "bug" occurs on a supported platform (ensure Platforms in OP are ✅)
• This bug is not a duplicate report (check E/App issues and #expensify-bugs)
  • If it is, comment with a link to the original report, close the issue and add any novel details to the original issue instead
• This bug is reproducible using the reproduction steps in the OP. S/O
  • If the reproduction steps are clear and you're unable to reproduce the bug, check with the reporter and QA first, then close the issue.
  • If the reproduction steps aren't clear and you determine the correct steps, please update the OP.
• This issue is filled out as thoroughly and clearly as possible
  • Pay special attention to the title, results, platforms where the bug occurs, and if the bug happens on staging/production.
• I have reviewed and subscribed to the linked Slack conversation to ensure Slack/Github stay in sync

[melvin-bot]

Triggered auto assignment to Contributor-plus team member for initial proposal review - @parasharrajat (External)

[wlegolas]

Proposal

Please re-state the problem that we are trying to solve in this issue.

mWeb - Chat - Tapping on new messages after marking a message unread leads to a blank page

What is the root cause of that problem?

There are two problems:

1- Showing a blank screen when clicking on the New Message button

The problem is related to the InvertedFlatList component because when the user scrolls and this action is not finished, when the user clicks on the "New messages" button, the action will scroll to the Report view bottom, but the scroll action is still occurring, that way the InvertedFlatList is not ready to show the information and show a blank screen.

Here's the code in the InvertedFlatList that handles the scrolls, you can see there is a delay of 250ms to finish the scroll action and this code was created to deal with a scroll for Web application because the react-native-web doesn't support the controlled props for the scroll.

App/src/components/InvertedFlatList/index.js

Lines 82 to 102 in 72e73ae

	const handleScroll = (event) => {
	onScroll(event);
	const timestamp = Date.now();
	if (scrollEndTimeout.current) {
	clearTimeout(scrollEndTimeout.current);
	}
	if (lastScrollEvent.current) {
	scrollEndTimeout.current = setTimeout(() => {
	if (lastScrollEvent.current !== timestamp) {
	return;
	}
	// Scroll has ended
	lastScrollEvent.current = null;
	onScrollEnd();
	}, 250);
	}
	lastScrollEvent.current = timestamp;
	};

In the InvertedFlatList component there is the internal method onScroll that is called when the scroll starts here, that's the root cause to see a blank screen.

If you scroll and wait for this action to finish and click on the "New messages" button, this behavior won't occur.

2- When clicking on the "New Message" button the scroll goes to the view bottom

The FloatingMessageCounter component receives in the property onClick the action to be executed when clicking on the floating message element, the ReportActionsList component uses the FloatingMessageCounter and passes to the onClick property the method scrollToBottomAndMarkReportAsRead, this method will always scrolls to the bottom:

App/src/pages/home/report/ReportActionsList.js

Lines 303 to 307 in 8c909d1

	const scrollToBottomAndMarkReportAsRead = () => {
	reportScrollManager.scrollToBottom();
	readActionSkipped.current = false;
	Report.readNewestAction(report.reportID);
	};

What changes do you think we should make in order to solve the problem?

1- Showing a blank screen when clicking on the New Message button

We should call the props.onScroll inside the InvertedFlatList component only when the scroll action ends. To call the props.onScroll when finishing the scroll we should make three changes:

1. Change the method onScrollEnd to receive one parameter event with the scroll event, and call the props.onScroll(event);

For example:

/**
 * Emits when the scrolling has ended.
 * 
 * @param {Event} event - The onScroll event from the FlatList
 */
const onScrollEnd = (event) => {
    props.onScroll(event);

    eventHandler.current = DeviceEventEmitter.emit(CONST.EVENTS.SCROLLING, false);
    updateInProgress.current = false;
};

2. Change the method onScroll to not receive the parameter event and not call the props.onScroll(event);

For example:

/**
 * Emits when the scrolling is in progress. Also,
 * invokes the onScroll callback function from props.
 */
const onScroll = () => {
    if (updateInProgress.current) {
        return;
    }

    updateInProgress.current = true;
    eventHandler.current = DeviceEventEmitter.emit(CONST.EVENTS.SCROLLING, true);
};

3. Change the method handleScroll to pass the event object when calling the onScrollEnd method.

For example:

const handleScroll = (event) => {
    onScroll(); // <- Here we can remove the event from the parameter
    const timestamp = Date.now();

    if (scrollEndTimeout.current) {
        clearTimeout(scrollEndTimeout.current);
    }

    if (lastScrollEvent.current) {
        scrollEndTimeout.current = setTimeout(() => {
            if (lastScrollEvent.current !== timestamp) {
                return;
            }
            // Scroll has ended
            lastScrollEvent.current = null;
            onScrollEnd(event); // <- Here we need to change
        }, 250);
    }

    lastScrollEvent.current = timestamp;
};

2- When clicking on the New Message button the scroll goes to the view bottom

We should improve the method scrollToBottomAndMarkReportAsRead to find the index of the first unread message and use the scrollToIndex method from the reportScrollManager to set the scrolls to the first unread message using the found index.

For example,

const getScrollIndex = () => {
    const bottomIndex = 0;
    const firstUnreadMessageIndex = _.findIndex(sortedReportActions, shouldDisplayNewMarker);

    return firstUnreadMessageIndex > -1 ? firstUnreadMessageIndex : bottomIndex;
};

const scrollToBottomAndMarkReportAsRead = () => {
    const scrollIndex = getScrollIndex();

    reportScrollManager.scrollToIndex(scrollIndex, false);
    readActionSkipped.current = false;
    Report.readNewestAction(report.reportID);
};

Note: maybe we should rename the method name from scrollToBottomAndMarkReportAsRead to scrollToNextUnreadMessageAndMarkReportAsRead

What alternative solutions did you explore? (Optional)

1- Showing a blank screen when clicking on the "New Message" button

Besides my proposal above, we can also solve this issue with one of these approaches:

Approach 1: Creating a mechanism to delay the onClick in the FloatingMessageCounter component

We should change the FloatingMessageCounter component to have a mechanism to call the onClick property when the scroll is not occurring.

We can use almost the same approach that the Hoverable component does, adding an event listener to the scrolling event and setting the value if the scroll is occurring or not into a memoized value, this memoized value will be used to handle the onClick, if there is an active scroll we can delay the click.

App/src/components/Hoverable/index.tsx

Lines 83 to 96 in 72e73ae

	useEffect(() => {
	if (!shouldHandleScroll) {
	return;
	}
	const scrollingListener = DeviceEventEmitter.addListener(CONST.EVENTS.SCROLLING, (scrolling) => {
	isScrolling.current = scrolling;
	if (!scrolling) {
	setIsHovered(isHoveredRef.current);
	}
	});
	return () => scrollingListener.remove();
	}, [shouldHandleScroll]);

Approach 2: Adding a delay to call the onClick method

Changing FloatingMessageCounter component to have a mechanism for delaying the onClick using the value from CONST.ANIMATED_TRANSITION that is greater than 250 that is used here

POC

poc-issue-31591.mp4

[parasharrajat]

@wlegolas Please explain how the posted root cause caused this issue.

[wlegolas]

Hi @parasharrajat

Sorry, I focused only on the solution to scroll to the first unread message.

I updated my proposal with this root cause and the changes that we need to make to fix it.

If you have any questions, please feel free to contact me

[wlegolas]

Proposal

Updated