Limit fallback image rendering and add priority to reduce carousel lo… by szymonzalarski98 · Pull Request #82282 · Expensify/App

szymonzalarski98 · 2026-02-12T16:32:12Z

Explanation of Change

Previously, Lightbox rendered a fallback for every carousel page outside the lightbox window. In a 50-image carousel, this meant ~47 unnecessary image downloads queued simultaneously, starving the active image of network bandwidth (SDWebImage processes downloads FIFO with 3 concurrent slots).

This PR adds two fixes:

Bounded fallback window — limits fallback image rendering to ±2 pages around the active page (FALLBACK_OFFSET), reducing total mounted components from ~50 to ~5.
Image loading priority — passes expo-image priority prop so the active image downloads first (high), adjacent lightbox pages next (normal), and fallback images last (low).

Fixed Issues

$ #81250
PROPOSAL:

Proposal: Eliminate Bandwidth Starvation in Attachment Carousel Loading

Background
The Expensify mobile application uses a Lightbox component to display full-screen expense receipts and attachments in a horizontally swipeable carousel. To enable smooth swiping between images, the carousel renders the current attachment as well as several adjacent pages.

Problem
When a user opens an attachment carousel in a chat page, if the application simultaneously initiates multiple attachments downloads for all adjacent carousel items, then the active attachment's download speed is throttled by network contention, resulting in a several-second delay before the user can read the attachment.

Solution
We will implement intelligent image loading prioritization and bandwidth management to ensure the active attachment always has the highest throughput.

Introduce Image Priority: Add a priority prop to the base Image component that accepts 'low', 'normal', or 'high'.Limit Fallback Render Range: Introduce a range constraint (FALLBACK_OFFSET = 2) that prevents Lightbox from rendering its fallback component for pages more than two positions away from the user's current view. The Lightbox component itself remains mounted on all pages, but distant ones simply do not render an Image — and therefore do not initiate any network requests.Dynamic Priority Assignment:Assign High priority to the active page.Assign Normal priority to pages within the lightbox window (±1 on iOS, ±0 on Android).Assign Low priority to pages within the fallback range (±2) but outside the lightbox window.Pages outside the fallback range render no Image component at all.Conditional Rendering: Update the Lightbox logic to strictly enforce that distant carousel items do not mount their Image components until they move within the permitted look-ahead range, using both a synchronous useMemo guard and an asynchronous state flag for defense-in-depth during fast swipe sequences.

Tests

Open a chat room that has many image attachments (30+)
Tap on the first image in the conversation to open the attachment carousel — verify the image loads within 1-2 seconds
Close the carousel and navigate away from the chat, hard refresh the app, then re-enter it
Tap on an image in the middle of the conversation — verify the image loads within 1-2 seconds
Close the carousel and navigate away from the chat, hard refresh the app, then re-enter it
Tap on the last image in the conversation — verify the image loads within 1-2 seconds (previously took 4-5+ seconds for distant images)
In each case, swipe left/right — adjacent images should appear immediately or with minimal delay
Verify zoom/pan gestures work correctly on the active image
Verify no errors appear in the JS console

Offline tests

QA Steps

Same as tests

PR Author Checklist

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

images.1.mp4

iOS: mWeb Safari

MacOS: Chrome / Safari

Images-web.1.mp4

…ad times

codecov · 2026-02-12T16:39:45Z

Codecov Report

✅ Changes either increased or maintained existing code coverage, great job!

Files with missing lines	Coverage Δ
src/CONST/index.ts	`87.39% <ø> (ø)`
src/components/Lightbox/index.tsx	`88.52% <100.00%> (+23.81%)`	⬆️
... and 565 files with indirect coverage changes

VickyStash · 2026-02-12T18:21:54Z

src/components/Image/types.ts

+    /** Priorities for completing loads. If more than one load is queued at a time,
+     *  the load with the higher priority will be started first.
+     *  Maps to SDWebImageHighPriority (iOS) and Glide.Priority.IMMEDIATE (Android). */
+    priority?: 'low' | 'normal' | 'high' | null;


Maybe we can add a const with these values, so it's easier to use?

Thank you, very good comment, I've updated my PR according to your suggestions

LukasMod · 2026-02-13T07:28:33Z

src/components/Lightbox/index.tsx

+    // Limits fallback image rendering to only a few pages around the active page.
+    // This prevents distant carousel items from queuing unnecessary image downloads,
+    // which would starve the active image of network bandwidth.
+    const isFallbackInRange = useMemo(() => {


Lightbox is already compiled with react compiler. Maybe we don't need useMemo here?

You're right, thanks

LukasMod · 2026-02-13T07:29:37Z

src/components/Lightbox/index.tsx

        [onScaleChangedContext, onScaleChangedProp],
    );

+    const imagePriority = useMemo(() => {


Same. Is useMemo necessary here as its already using React compiler?

You're right, thanks

fabioh8010 · 2026-02-13T11:57:02Z

tests/unit/LightboxTest.tsx

I suppose this is still WIP, right? I'm seeing several errors

Yes, but today I'm going to fully publish ready to review PR

PR is ready for review :)

melvin-bot · 2026-02-16T06:33:21Z

@abzokhattab 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]

github-actions · 2026-02-16T06:37:14Z

tests/unit/LightboxTest.tsx

+        });
+    }
+    return {
+        // eslint-disable-next-line @typescript-eslint/naming-convention


❌ CONSISTENCY-5 (docs)

The eslint-disable-next-line comment here lacks a justification explaining why the rule is disabled. While __esModule: true is a common pattern in Jest mocks that requires the naming convention rule to be disabled, please add a brief comment explaining the reason.

// eslint-disable-next-line @typescript-eslint/naming-convention -- __esModule is required by Jest to properly mock ES modules with default exports __esModule: true,

Please rate this suggestion with 👍 or 👎 to help us improve! Reactions are used to monitor reviewer efficiency.

github-actions · 2026-02-16T06:37:19Z

tests/unit/LightboxTest.tsx

+    return {
+        // eslint-disable-next-line @typescript-eslint/naming-convention
+        __esModule: true,
+        default: ({children}: {children: React.ReactNode}) => MockReact.createElement(View, {testID: 'multi-gesture-canvas'}, children),


❌ CONSISTENCY-5 (docs)

The eslint-disable-next-line comment here lacks a justification explaining why the rule is disabled. Please add a brief comment explaining the reason.

// eslint-disable-next-line @typescript-eslint/naming-convention -- __esModule is required by Jest to properly mock ES modules with default exports __esModule: true,

Please rate this suggestion with 👍 or 👎 to help us improve! Reactions are used to monitor reviewer efficiency.

joekaufmanexpensify · 2026-02-16T15:04:41Z

No product review needed

cristipaval · 2026-02-16T21:00:28Z

Waiting for @abzokhattab to review and test

abzokhattab · 2026-02-16T22:00:33Z

tests/unit/LightboxTest.tsx

Looks like we are missing tests for the NORMAL priority

Thanks, added them in the latest commit

abzokhattab · 2026-02-16T23:47:07Z

Reviewer Checklist

I have verified the author checklist is complete (all boxes are checked off).
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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
- I verified all numbers, amounts, dates and phone numbers shown in the product are using the localization methods
- I verified any copy / text that was added to the app is grammatically correct in English. It adheres to proper capitalization guidelines (note: only the first word of header/labels should be capitalized), and is either coming verbatim from figma or has been approved by marketing (in order to get marketing approval, ask the Bug Zero team member to add the Waiting for copy label to the issue)
- I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
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Screenshots/Videos

Android: HybridApp

Screen.Recording.2026-02-17.at.00.36.06.mov

Screen.Recording.2026-02-17.at.00.38.04.mov

Android: mWeb Chrome

Screen.Recording.2026-02-17.at.00.42.03.mov

iOS: HybridApp

Untitled.mov

Screen.Recording.2026-02-17.at.00.16.03.mov

Screen.Recording.2026-02-17.at.00.16.20.mov

iOS: mWeb Safari

MacOS: Chrome / Safari

Screen.Recording.2026-02-17.at.00.19.38.mov

Untitled.mov

abzokhattab · 2026-02-16T23:48:30Z

Nice Improvement @szymonzalarski98 i added a small comment on the tests but the manual testing looks good to me ... i uploaded 30 big images ranging from 5mg to 10mg each and the loading time improved

github-actions · 2026-02-17T10:21:29Z

🚧 @cristipaval has triggered a test Expensify/App build. You can view the workflow run here.

github-actions · 2026-02-17T10:25:06Z

🧪🧪 Use the links below to test this adhoc build on Android, iOS, and Web. Happy testing! 🧪🧪
Built from App PR #82282.

Android 🤖	iOS 🍎
https://ad-hoc-expensify-cash.s3.us-east-1.amazonaws.com/rock-artifacts/ad-hoc/rock-android-Adhoc-fda1e22-db0883a-615aea29a71152fbce6a042c833c31a401b6fdf2/index.html	https://ad-hoc-expensify-cash.s3.us-east-1.amazonaws.com/rock-artifacts/ad-hoc/rock-ios-device-AdHoc-fda1e22-db0883a-92348e39bcc6b871f545d72c214f8c32e632b792/index.html

Web 🕸️
https://82282.pr-testing.expensify.com

👀 View the workflow run that generated this build 👀

OSBotify · 2026-02-17T10:28:53Z

✋ This PR was not deployed to staging yet because QA is ongoing. It will be automatically deployed to staging after the next production release.

OSBotify · 2026-02-18T00:07:16Z

🚀 Deployed to staging by https://github.com/cristipaval in version: 9.3.21-0 🚀

platform	result
🕸 web 🕸	success ✅
🤖 android 🤖	success ✅
🍎 iOS 🍎	success ✅

…nternal/szymonzalarski/attachments/images-load-extremely-slow-when-viewing-in-the-app" This reverts commit fda1e22, reversing changes made to d3c00a5.

OSBotify · 2026-02-18T22:02:50Z

🚀 Deployed to production by https://github.com/mountiny in version: 9.3.21-4 🚀

platform	result
🕸 web 🕸	success ✅
🤖 android 🤖	success ✅
🍎 iOS 🍎	success ✅

OSBotify · 2026-02-18T22:20:56Z

🚀 Deployed to staging by https://github.com/cristipaval in version: 9.3.22-0 🚀

platform	result
🕸 web 🕸	success ✅
🤖 android 🤖	cancelled 🔪
🍎 iOS 🍎	cancelled 🔪

OSBotify · 2026-02-19T17:21:45Z

🚀 Deployed to production by https://github.com/mountiny in version: 9.3.22-4 🚀

platform	result
🕸 web 🕸	cancelled 🔪
🤖 android 🤖	success ✅
🍎 iOS 🍎	cancelled 🔪

OSBotify · 2026-02-19T17:36:05Z

🚀 Deployed to production by https://github.com/mountiny in version: 9.3.22-4 🚀

platform	result
🕸 web 🕸	failure ❌
🤖 android 🤖	success ✅
🍎 iOS 🍎	cancelled 🔪

Limit fallback image rendering and add priority to reduce carousel lo…

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melvin-bot bot assigned szymonzalarski98 Feb 12, 2026

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[Due for payment 2026-03-06] [$250] [Performance] Attachments/images load extremely slowly when viewing in the app #81250

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VickyStash reviewed Feb 12, 2026

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Use constant variables for image props

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PR fixes and test added

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Lint and prettier fixes

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szymonzalarski98 marked this pull request as ready for review February 16, 2026 06:33

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Fix consistency in eslint comments

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szymonzalarski98 changed the title ~~[DRAFT] Limit fallback image rendering and add priority to reduce carousel lo…~~ Limit fallback image rendering and add priority to reduce carousel lo… Feb 16, 2026

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melvin-bot bot requested a review from cristipaval February 16, 2026 23:48

Add test cases for normal priority

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cristipaval merged commit fda1e22 into Expensify:main Feb 17, 2026
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OSBotify mentioned this pull request Feb 17, 2026

Deploy Checklist: New Expensify 2026-02-17 #82716

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mitarachim mentioned this pull request Feb 18, 2026

[Due for payment 2026-02-26] Android - Attachment - Blank preview on the 4th attachment preview #82723

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cristipaval mentioned this pull request Feb 18, 2026

[CP Staging] Revert "Limit fallback image rendering and add priority to reduce carousel load times" #82837

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szymonzalarski98 mentioned this pull request Feb 23, 2026

Attachments carousel loading optimizations #83185

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