[Sentry: APP-KDF] HybridApp iOS fatal hang — RCTMarkdownUtils applyMarkdownFormatting blocks the main thread in the Yoga measure pass

@Krishna2323

Sentry

https://expensify.sentry.io/issues/APP-KDF

Impact (snapshot at filing)

Users (total since first seen): 37
Events: 42
Users (last 14d): 37
First seen: 2026-08-24 (Sentry substatus is still new)
Last seen: ongoing
Platform: iOS HybridApp
App version(s): 9.4.56-3, 9.4.57-3, 9.4.58-4, 9.4.59-5, 9.4.60-2
Mechanism: AppHang (Fatal App Hang Fully Blocked)
Culprit: -[RCTMarkdownUtils applyMarkdownFormatting:withDefaultTextAttributes:markdownStyle:parserId:]

Every affected user in the 14 day window is a new user of this signature, so this is a fresh
regression rather than a long tail.

Stack trace (top frames, main thread)

__ulock_wait2
_os_unfair_lock_lock_slow
objc_sync_enter
-[RCTMarkdownUtils applyMarkdownFormatting:withDefaultTextAttributes:markdownStyle:parserId:]
facebook::react::MarkdownTextInputDecoratorShadowNode::applyMarkdownFormattingToTextInputState
facebook::react::MarkdownTextInputDecoratorShadowNode::measureContent
facebook::react::MarkdownTextInputDecoratorShadowNode::yogaNodeMeasureCallbackConnector
facebook::yoga::Node::measure (Node.cpp:59)
facebook::yoga::measureNodeWithMeasureFunc (CalculateLayout.cpp:356)
facebook::yoga::calculateLayoutImpl (CalculateLayout.cpp:1349)
... ~100 further Yoga frames: calculateLayoutInternal / calculateLayoutImpl /
    computeFlexBasisForChild / resolveFlexibleLength / layoutAbsoluteDescendants ...
facebook::react::YogaLayoutableShadowNode::layoutTree (YogaLayoutableShadowNode.cpp:683)

The event also shows LiveMarkdownRuntime_queue and LiveMarkdownRuntime(timeout) threads alive
while the main thread sits in the lock.

Suspected cause

The Yoga measure callback for MarkdownTextInputDecoratorShadowNode calls
applyMarkdownFormatting synchronously on the main thread, and that method blocks on the
@synchronized lock inside RCTMarkdownUtils. The stack shows a very deep layout pass, roughly 100
Yoga frames with repeated layoutAbsoluteDescendants recursion, so the measure function is invoked
many times in one pass while the parser lock is held elsewhere. The main thread waits past the two
second watchdog threshold and iOS terminates the app.

Two things worth checking:

Whether applyMarkdownFormatting can be made lock free on the measure path, or given a cached
measurement so repeated measures of unchanged text do not re-enter the parser.
Why the layout tree is this deep. The repeated absolute-position recursion suggests a markdown
input nested inside several absolutely positioned containers, which multiplies measure calls.

Reproduction

Unknown — see the events linked from the Sentry issue. The affected sample includes iPad on iOS 26.6,
so a large composer with long markdown content is a plausible starting point.

[$250] [Sentry: APP-EF1] HybridApp iOS App Hang in MarkdownTextInput Yoga layout (createStringFromUtf8) is the same component and the same Yoga measure path, but a different throw site. That issue is closed and paid, so treat it as prior art for the area rather than a duplicate.
Parent tracker: Top Sentry crashes monthly check-in

Issue Owner

Current Issue Owner: @Krishna2323

@mountiny

@mountiny Uh oh! This issue is overdue by 2 days. Don't forget to update your issues!

@mountiny

@mountiny Uh oh! This issue is overdue by 2 days. Don't forget to update your issues!

@mountiny

@mountiny Uh oh! This issue is overdue by 2 days. Don't forget to update your issues!

@Krishna2323

Triggered auto assignment to Contributor-plus team member for initial proposal review - @Krishna2323 (External)

Unable to auto-create job on Upwork. The BZ team member should create it manually for this issue.

@Krishna2323

@Krishna2323 Eep! 4 days overdue now. Issues have feelings too...

@MelvinBot

@MelvinBot can you check this event in Sentry? Is it still reproducible in the latest versions?

Short answer: this should already be fixed as of App 9.4.65 — but I could not read Sentry to confirm no new events have landed since. My Sentry connection failed on this run, so everything below is from the code and version history, not from live event data.

What I found

Every affected version in the report (9.4.56-3 → 9.4.60-2) pinned @expensify/react-native-live-markdown at 0.1.334, which does not contain the fix for this exact hang.

The fix is Fix main thread hang on iOS (#772), merged 2026-08-24. It targets precisely the stack in this issue — main thread stuck in objc_sync_enter inside applyMarkdownFormatting:…markdownStyle:parserId: during the Yoga measure pass. Both the lock inversion and the "measure parses synchronously" problem described in the issue body are addressed:

The @synchronized block now covers only the _markdownStyle/_parserId writes, so no lock is held across the worklet runtime call (apple/RCTMarkdownUtils.mm:48-51).
On a main-thread cache miss it warms the cache asynchronously and returns instead of entering the runtime (apple/RCTMarkdownUtils.mm:56-75).
A 4-entry parse cache replaced the single-entry memo, so repeated measures of unchanged text no longer re-enter the parser (apple/MarkdownParser.mm).

Version chain

	live-markdown	Has fix?
App 9.4.56-3 … 9.4.64-3	0.1.334	❌
App 9.4.65-0 onward	0.1.336	✅
App main today (9.4.75-1)	0.1.336	✅

The bump landed in #99324 (merged 2026-08-29), first shipping in 9.4.65-0.

Note the App-side patch PR referenced from #772 (#97871) was closed unmerged — the fix went upstream into the library instead and reached App via the version bump, so the coverage is real.

What still needs a human

Filter the Sentry issue to release >= 9.4.65. If APP-KDF has zero events on 9.4.65+, close it. If events are still arriving on those releases, the fix is incomplete and this needs reopening as a follow-up on Update version to 1.0.1-174 #772.
I did not run a live repro. This is an iOS native watchdog hang; the tester available to me on this run drives web, which cannot reproduce or rule out an Objective-C lock stall on the iOS main thread.

The linked prior-art issue #93831 (APP-EF1) is the same root cause — #772 names it directly — so both should clear together.

Next Steps: Reply with @MelvinBot <your feedback> to refine this analysis.

view run · view recording

@Krishna2323

@Krishna2323 Still overdue 6 days?! Let's take care of this!

I'll review this today.

@Krishna2323

@Krishna2323 Whoops! This issue is 2 days overdue. Let's get this updated quick!

@Krishna2323

@Krishna2323 Eep! 4 days overdue now. Issues have feelings too...

@mountiny

@mountiny Not reproducible on current builds, this is already fixed. All 59 events are on 9.4.64-3 or older, with zero events on 9.4.65+ despite those releases being in the wild (9.4.65-3 has 49 other iOS AppHangs in the last 7d).

Fixed by 982c10c, bumping @expensify/react-native-live-markdown 0.1.334 → 0.1.336, which narrowed the @synchronized (self) block in RCTMarkdownUtils.applyMarkdownFormatting to just the two field assignments and stopped the Yoga measure path from parsing on the main thread. This was the fix for #93831 (APP-EF1), which turns out to be the same root cause as this one, rather than just prior art.

Thanks for looking into this, we can close this one then

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[Sentry: APP-KDF] HybridApp iOS fatal hang — RCTMarkdownUtils applyMarkdownFormatting blocks the main thread in the Yoga measure pass #99849

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[Sentry: APP-KDF] HybridApp iOS fatal hang — RCTMarkdownUtils applyMarkdownFormatting blocks the main thread in the Yoga measure pass #99849

Description

Sentry

Impact (snapshot at filing)

Stack trace (top frames, main thread)

Suspected cause

Reproduction

Related

Activity

melvin-bot commented on Sep 2, 2026

melvin-bot commented on Sep 3, 2026

melvin-bot commented on Sep 3, 2026

melvin-bot commented on Sep 3, 2026

melvin-bot commented on Sep 3, 2026

melvin-bot commented on Sep 8, 2026

mountiny commented on Sep 10, 2026

MelvinBot commented on Sep 10, 2026

What I found

Version chain

What still needs a human

melvin-bot commented on Sep 10, 2026

Krishna2323 commented on Sep 11, 2026

melvin-bot commented on Sep 14, 2026

melvin-bot commented on Sep 16, 2026

Krishna2323 commented on Sep 18, 2026

mountiny commented on Sep 18, 2026

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