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Mobile: fix the playlist reorder crash when the list changes mid-drag (LUMEN-4) - #83
Fixes Sentry LUMEN-4: a fatal, unhandled TypeError: Cannot read property 'key' of undefined on the playlist detail screen (mobile/app/(tabs)/(playlists)/[id].tsx).
Why it crashed
react-native-reorderable-list 0.18.0 reports a drop as indices into the rows it showed when the drag began, about one drop animation after the finger lifts. On the way, reorder → markCells (ReorderableListCore.tsx:494-518, called via runOnJS at :698) calls keyExtractor(data[i], i) for every index between the two, with no bounds check. Indices are only clamped to the item count while the drag is moving, never at the drop.
In reorder mode the list's rows can change mid-drag: the refetch after the previous drag's save, a focus refetch, or the TIDAL auto-download refetchInterval. When the list got shorter, the drop's indices ran past the end, so the shared keyExtractor read undefined.key inside a runOnJS callback, which reaches the global handler as a fatal. When rows only shifted, the drop moved the wrong track.
FlashList 2.0.2 (the read-only list) processes data updates synchronously in render and has maintainVisibleContentPosition disabled here, so it isn't the caller. Its keyExtractor gets the same guard anyway.
Changes
components/steady-reorderable-list.tsx: wraps ReorderableList. From drag start (onDragStart worklet) until the drop is handled, it keeps showing the rows the drag began on. Then it replays the drop onto the latest data by row key and calls onReorder with that move. A drop on the same slot lets go in onDragEnd, because that drop never reaches onReorder. The drop handler reads the latest render through a ref, because the library calls the onReorder it captured when the finger lifted.
lib/reorder-drop.ts:
rebaseDrop: the dragged row goes under the row it was dropped under. If that row is gone, or the drop was at the top, it goes above the row it was dropped above. A drop whose row (or both neighbours) is gone reports nothing.
rowKeyOrIndex: the keyExtractor, with an index fallback for a missing row.
playlistAfterDrop: movePlaylistEntry that returns null for out-of-range indices.
Playlist screen:
Uses the steady list.
Pauses the auto-download refetchInterval while reorder mode is on.
onReorder ignores from/to outside localTracks, so a reorder can never send undefined track ids.
The override/retire semantics are unchanged.
The change is JS-only (no native, plugin or patch changes), so it can ship as an OTA.
Verification
New tests:
tests/reorder-drop.test.ts: the rebase cases, plus an exhaustive check that every drop against every shrunken list reports only in-range moves of the dragged row; out-of-range indices; the key fallback.
tests/steady-reorderable-list.test.ts: drives the wrapper through the library's drag → refetch → drop sequence with a small hook harness, as in playlist-picker-marks-wait.test.ts.
With the hold removed, the LUMEN-4 test fails with exactly Cannot read properties of undefined (reading 'key').
cd mobile, each run as a separate step: npm test (51 files, 461 tests), npm run typecheck, npm run lint. All exit 0.
npx expo export --platform ios (production, with and without bytecode) succeeds. In the unminified bundle, onDragStart/onDragEnd compile to worklets (__workletHash) and are memoized once by the React Compiler.
… (LUMEN-4)
Sentry LUMEN-4 was a fatal "Cannot read property 'key' of undefined" in
the playlist screen. react-native-reorderable-list 0.18 reports a drop as
indices into the rows it showed when the drag began, and before that its
markCells reads keyExtractor(data[i]) for every index between the two,
with no bounds check against the current data. A refetch that shortened
the list mid-drag (the save after the previous drag, a focus refetch, the
TIDAL auto-download poll) left those indices past the end, so the shared
keyExtractor read `undefined.key` inside a runOnJS callback. A refetch
that only shifted rows moved the wrong track.
- SteadyReorderableList wraps ReorderableList: from drag start until the
drop is handled it keeps showing the rows the drag began on, then
replays the drop onto the latest data by row key (rebaseDrop) and
reports that move. A drop whose row is gone reports nothing.
- The auto-download refetchInterval pauses while reorder mode is on.
- keyExtractor falls back to the index for a missing row.
- onReorder ignores indices outside localTracks (playlistAfterDrop), so
a reorder can't send undefined track ids.
The override/retire semantics are unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Commits
Reviewing files that changed from the base of the PR and between 008f303 and 256f4da.
Files selected for processing: 5
mobile/app/(tabs)/(playlists)/[id].tsx
mobile/components/steady-reorderable-list.tsx
mobile/lib/reorder-drop.ts
mobile/tests/reorder-drop.test.ts
mobile/tests/steady-reorderable-list.test.ts
Walkthrough
The playlist screen now uses a reorderable list that retains rows during a drag and rebases the drop against current data. Reorder helpers validate indices and playlist entries. Auto-download polling pauses during reorder mode.
Changes: Playlist Reordering
Layer / File(s)
Summary
Reorder validation and rebasing mobile/lib/reorder-drop.ts, mobile/tests/reorder-drop.test.ts
Helpers validate drop indices, map dragged rows to current data, and return valid playlist reorder results. Tests cover changed rows, invalid moves, and retained server position slots.
Retain rows and rebase drops mobile/components/steady-reorderable-list.tsx, mobile/tests/steady-reorderable-list.test.ts
The list holds drag-start rows, rebases drops onto current data, and uses the latest reorder callback. Tests cover refetch changes, missing rows, and no-op drops.
Connect the playlist screen mobile/app/(tabs)/(playlists)/[id].tsx
The screen uses the new list and shared row-key helper. It validates reorder results before saving and pauses auto-download polling during reorder mode.
sequenceDiagram
participant PlaylistScreen
participant SteadyReorderableList
participant rebaseDrop
participant playlistAfterDrop
PlaylistScreen->>SteadyReorderableList: Render current playlist rows
SteadyReorderableList->>rebaseDrop: Rebase drop against current rows
rebaseDrop-->>SteadyReorderableList: Return current indices or null
SteadyReorderableList->>PlaylistScreen: Report valid rebased indices
PlaylistScreen->>playlistAfterDrop: Validate and reorder playlist entries
playlistAfterDrop-->>PlaylistScreen: Return reordered entries or null
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Merge Risk: Low · up to 008f3
Reordering certain duplicate playlist entries can move the wrong row. This is a narrow, recoverable case, but row identity should be fixed or explicitly accepted before merging.
Pre-merge checks: 5
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Title check
The title clearly and concisely describes the main change: fixing the mobile playlist reorder crash when the list changes during a drag. The issue identifier is relevant.
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Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 5 files.
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I’m a rabbit with a list to arrange, I hold my rows while fresh ones change. A valid hop finds its place anew, A missing row gets no move through. Then polling rests till the drag is through.
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @mobile/app/(tabs)/(playlists)/[id].tsx:
- Around line 664-674: Update the row-model construction in the useMemo that
builds rowModels so duplicate entries receive distinct keys: count occurrences
of each playlistTrackKey(entry) and append a stable per-occurrence suffix. Keep
the occurrence order aligned with localTracks so rebaseDrop can identify the
dragged row correctly.
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Commits
Reviewing files that changed from the base of the PR and between e00c2fe and 008f303.
Files selected for processing: 5
mobile/app/(tabs)/(playlists)/[id].tsx
mobile/components/steady-reorderable-list.tsx
mobile/lib/reorder-drop.ts
mobile/tests/reorder-drop.test.ts
mobile/tests/steady-reorderable-list.test.ts
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…p settles
- One add can save a track twice with the same added_at and adder, so
their row keys matched and a drop moved the first copy instead of the
dragged one. numberRepeatedKeys suffixes repeats in list order.
(CodeRabbit)
- The hold now lets go a little after the drop animation, unless
onReorder arrives first. The library calls onReorder neither for a row
put back in its slot nor when the finger lifts exactly on the new
slot. The first case released mid-animation; the second left the
list holding stale rows until the next drop. The rows a drag's
indices refer to sit in a ref, so a late drop still replays
correctly, and a new drag cancels the previous drag's timer.
(Fable round 1, two lows)
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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What
Fixes Sentry LUMEN-4: a fatal, unhandled
TypeError: Cannot read property 'key' of undefinedon the playlist detail screen (mobile/app/(tabs)/(playlists)/[id].tsx).Why it crashed
react-native-reorderable-list0.18.0 reports a drop as indices into the rows it showed when the drag began, about one drop animation after the finger lifts. On the way,reorder→markCells(ReorderableListCore.tsx:494-518, called viarunOnJSat:698) callskeyExtractor(data[i], i)for every index between the two, with no bounds check. Indices are only clamped to the item count while the drag is moving, never at the drop.In reorder mode the list's rows can change mid-drag: the refetch after the previous drag's save, a focus refetch, or the TIDAL auto-download
refetchInterval. When the list got shorter, the drop's indices ran past the end, so the sharedkeyExtractorreadundefined.keyinside arunOnJScallback, which reaches the global handler as a fatal. When rows only shifted, the drop moved the wrong track.FlashList 2.0.2 (the read-only list) processes data updates synchronously in render and has
maintainVisibleContentPositiondisabled here, so it isn't the caller. ItskeyExtractorgets the same guard anyway.Changes
components/steady-reorderable-list.tsx: wrapsReorderableList. From drag start (onDragStartworklet) until the drop is handled, it keeps showing the rows the drag began on. Then it replays the drop onto the latestdataby row key and callsonReorderwith that move. A drop on the same slot lets go inonDragEnd, because that drop never reachesonReorder. The drop handler reads the latest render through a ref, because the library calls theonReorderit captured when the finger lifted.lib/reorder-drop.ts:rebaseDrop: the dragged row goes under the row it was dropped under. If that row is gone, or the drop was at the top, it goes above the row it was dropped above. A drop whose row (or both neighbours) is gone reports nothing.rowKeyOrIndex: thekeyExtractor, with an index fallback for a missing row.playlistAfterDrop:movePlaylistEntrythat returns null for out-of-range indices.refetchIntervalwhile reorder mode is on.onReorderignoresfrom/tooutsidelocalTracks, so a reorder can never send undefined track ids.The change is JS-only (no native, plugin or patch changes), so it can ship as an OTA.
Verification
tests/reorder-drop.test.ts: the rebase cases, plus an exhaustive check that every drop against every shrunken list reports only in-range moves of the dragged row; out-of-range indices; the key fallback.tests/steady-reorderable-list.test.ts: drives the wrapper through the library's drag → refetch → drop sequence with a small hook harness, as inplaylist-picker-marks-wait.test.ts.Cannot read properties of undefined (reading 'key').cd mobile, each run as a separate step:npm test(51 files, 461 tests),npm run typecheck,npm run lint. All exit 0.npx expo export --platform ios(production, with and without bytecode) succeeds. In the unminified bundle,onDragStart/onDragEndcompile to worklets (__workletHash) and are memoized once by the React Compiler.🤖 Generated with Claude Code
Summary by CodeRabbit