Lightroom won't start

What happens: I launched LrC and opened my catalog. The app showed no window, and macOS reported it as “Application Not Responding” for a long time. The other catalog I use opens normally. This has happened before, and I’ve assumed LrC was frozen and force-quit it several times.

What it was actually doing: I looked at it with Activity Monitor, sample, and read-only queries against the catalog. LrC wasn’t frozen. At launch it was working through AgLibraryImageXMPUpdater, the internal queue for “write metadata to XMP”, and the main thread was blocked waiting for it to finish.

• About 15,000 photos were queued within 2–3 minutes of the previous launch. I didn’t make any bulk changes. I edited a handful of photos in that session.
• Each queued photo means reading the file over SMB, at about 4–5 photos a second. That works out to roughly an hour of startup with no window.
• Not one file on disk was actually changed. Every check found the metadata already matched. The whole hour went on confirming nothing needed to be written.
• The only trace was HangMonitorThread: main thread pulse not received in the console log, once a minute. No progress bar, no status message, no log line saying what it was doing.

Why this is a problem:

1. “Not Responding” while busy looks exactly like a real hang, so people force-quit, which throws away progress and restarts the work on the next launch.
2. Housekeeping like this shouldn’t stop the catalog opening. It already runs in the background while you work, so there’s no reason it has to finish before the window can appear.
3. Nothing explains why 15,000 unchanged photos were queued in the first place.

What I’m asking for:

• Show the window first and run the XMP queue in the background, the way it already runs during a normal session.
• If something really has to finish before the window can open, show a progress dialog (“Checking metadata for 7,035 photos…”) with a cancel button.
• Log what triggered a mass queue, so users can tell why it happened.
• Clearer documentation that “Automatically write changes into XMP” rewrites JPEG/TIFF/DNG files in place rather than writing sidecars.

Has anyone else seen this? Is there a known trigger for LrC queuing XMP writes for thousands of photos that haven’t changed?