Lightroom Classic 15.3 hangs and must be force-quit — macOS spindump shows a self-deadlock in a Camera Raw thread (NegativeQueue) when opening RAW files in Develop or on import

Lightroom Classic 15.3 intermittently becomes completely unresponsive (permanent hang, requires Force Quit) when opening RAW images in the Develop module or when generating previews during import. A macOS `spindump` captured during a live hang shows the cause is a **self-deadlock inside the Camera Raw engine** bundled with Lightroom Classic 15.3.

The operating system's own deadlock detector flagged it. The spindump header reads:

- `Deadlocked: 1 thread - Thread 0x11612a "Queue Processor - NegativeQueue_1"`
- `Blocked by Deadlock: 3 threads - "Queue Processor - OpenNegativesForDevelop_1, _2, _3"`

The deadlock graph points the thread **at itself**:

`Deadlock: ... thread 0x11612a "NegativeQueue_1" -> thread 0x11612a "NegativeQueue_1"`

The deadlocked thread's call stack runs through AgKernel (Lua) into CameraRaw, ending at:

`... (CameraRaw) -> std::mutex::lock() -> _pthread_mutex_firstfit_lock_slow -> __psynch_mutexwait`

with the kernel annotation: `(blocked by turnstile waiting for this thread - part of a deadlock)`.

This is consistent with a **re-entrant acquisition of a non-recursive mutex** on the `NegativeQueue_1` worker thread inside Camera Raw. Once that thread self-deadlocks, the three `OpenNegativesForDevelop` threads (the Develop module opening the selected RAW) block on the same lock, and the main thread blocks on it as well when the app is asked to quit — which is why quitting also hangs and Force Quit is the only exit.

## Lightroom Classic Version Number

15.3 (15.3) — `CameraRaw.lrtoolkit` bundled engine reported as version 15.3

## OS Version Number

macOS 26.5 (build 25F71), arm64e

## Hardware

Mac Studio (Mac14,14), Apple M2 Ultra, 192 GB RAM

## Camera Model / File Types

Occurs with DJI Mavic 3 Cine DNG files and Sony mirrorless RAW (ARW) files. **Note:** the hang also occurs on files that were already imported and are stored on the internal SSD, with no camera, card, or external device connected — so it is not specific to any camera, card reader, or storage device.

## Step-by-Step Reproduction Instructions

This is intermittent (timing/race dependent), but the conditions that trigger it are consistent:

1. Open or create a catalog and import RAW files (DJI Mavic 3 Cine DNG and/or Sony ARW).
2. In the Library module, browse images normally — this works fine (cached previews render).
3. Switch to the Develop module on a RAW image (or perform an import that generates previews).
4. Intermittently — and more reliably after several catalog open/close cycles in one session (e.g., around the third catalog of a multi-shoot day) — the application hangs completely and stops responding.
5. The hang also reproduces with images already imported and stored on the local internal SSD, with no device connected.
6. Attempting File > Quit (or Cmd-Q) at that point also hangs; Force Quit is required.

In the captured incident, the application was unresponsive for approximately 16 minutes (958 seconds) before the spindump was taken.

## Expected Result

Switching to Develop (or importing) renders the RAW image and the application remains responsive. Quitting the application succeeds normally.

## Actual Result

The application becomes permanently unresponsive and must be Force Quit. macOS identifies a self-deadlock on the Camera Raw `NegativeQueue_1` thread.

## Diagnostic Evidence (attached)

- **`lrc_spindump.txt`** — full macOS spindump captured during the live hang. Contains the kernel deadlock determination, the self-pointing deadlock graph, and the full call stacks of the deadlocked thread and the three blocked Develop threads.
- **Activity Monitor process sample** — independent capture taken minutes earlier in the same hang, showing the `OpenNegativesForDevelop_1` thread and the main thread (during app termination) both blocked acquiring the same Camera Raw mutex.

Additional facts that rule out common environmental causes:

- **Not memory pressure:** process footprint at the time of the hang was ~1.56 GB on a 192 GB machine.
- **Not storage/disk I/O:** no read/open/file-system calls in any blocked stack; the blocking primitive is an in-memory mutex.
- **Not GPU:** no Metal/GPU frames in any live thread, and the hang also reproduces with Preferences > Performance > Use Graphics Processor set to **Off**.
- **Not catalog corruption:** reproduces in brand-new, empty catalogs.

## Regression Note

This began after updating to 15.3 (April 2026). Rolling Lightroom Classic back to **15.2.1** appears to avoid the hang so far, which suggests the self-deadlock is a regression introduced in 15.3.