Merge to HDR stalls on 15.4.1, persists after clean macOS/CC install (Tahoe 26.5.2)

Question

Description of Issue:Merge to HDR stalls (application unresponsive, beachball) when merging exposure-bracketed images on 15.4.1. Another user in an original 15.3 thread reported this resolved after updating to macOS Tahoe 26.5. I am on macOS Tahoe 26.5.2, a newer build, with a fresh OS install, fresh Creative Cloud install, and a brand-new catalog, and the stall still occurs on 15.4.1. This rules out the macOS-update fix reported by others for the 15.3 version of this issue.Lightroom Classic Version Number: 15.4.1 (Help > System Info)OS Version Number: macOS Tahoe 26.5.2Camera Model: Sony a7r V & DJI Mavic 4 ProComputer Version: Mac Studio M3 UltraStep-by-step Reproduction Instructions:Fresh macOS install, fresh CC/LrC install, no plugins, brand-new catalog.	Select a set of exposure-bracketed RAW images.	Photo > Photo Merge > HDR (or Ctrl+H or Ctrl+Shift+H).	Merge stalls partway through; have to quit to reset the merge process.Expected Result: HDR Merge completes and produces the merged DNG, including but not limited to adding to a stack.Actual Result: Lightroom Classic hangs during merge, requiring quit and restart. Disabling GPU acceleration (Preferences > Performance > Use Graphics Processor > Off) works around it but is NOT a fix.

johns40282744 · Answer

Update: I found a workaround that's holding on my Mac Studio system.The setting that matters isUse GPU for Preview Generation(Preferences > Performance), not the main Use Graphics Processor option.What I tested:Use Graphics Processoroffentirely: still stalled.Use Graphics Processoron, Preview Generation set toOff: 57 stacks merged clean with no stalling.Use Graphics Processoron, Preview Generation set toOn: stalled at 39 remaining out of 47 stacks.Same 47-stack batch re-run with Preview Generation set back toOff: completed cleanly.So Preview Generation set to Off is what resolved it for me, confirmed on the same batch that had just failed with it enabled. Setting it explicitly to On is also not a fix, which rules out the idea that Auto by itself was the problem.Other notes in case they help narrow this down:Stall points were inconsistent, sometimes as early as image 6, sometimes with 10 to 15 remaining.Batch size is not the trigger. I've had 20-stack batches stall and a 57-stack batch complete.The Create Stack checkbox made no difference either way as well.My system: Lightroom Classic 15.4.1, macOS Tahoe 26.5.2, Mac Studio M3 Ultra, fresh OS and Creative Cloud install, new catalog stored locally, images on an SMB share. Source files are a mix of Sony A7R V and DJI Mavic 4 Pro.My guess, unconfirmed, is GPU resource contention between background preview generation and the merge process, which would explain the random stall points.

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