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

[mountiny]

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:

1. 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.
2. 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.

Related

• [$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

[melvin-bot]

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

[melvin-bot]

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

[melvin-bot]

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

[melvin-bot]

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

[melvin-bot]

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

[melvin-bot]

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