useProgressAutoSave fires saveConfig ~once per second of reading. Two
write-time sources were doubling its IPC cost:
1. saveConfig wrote the WHOLE library.json (+ backup) on every call, so a
user with N books paid 2*JSON.stringify(N) per save. Chrome DevTools'
Bottom-Up profile on a release Android build showed processIpcMessage
chewing ~25% of main-thread time during a reading session.
2. nativeFileSystem.writeFile / copyFile defensively called plugin:fs|exists
before every write to ensure the parent dir existed. Same dir gets
probed once per save -- ~50% of IPC time per save was just exists()
round-trips against directories that have been there since the book
was opened.
Fix:
- LIBRARY_SAVE_THROTTLE_MS=30s coalesces a swipe burst into a single
library.json write. Per-book config.json is still written eagerly --
it's the sync source-of-truth and is small. flushPendingLibrarySave()
is called on hook unmount + window blur so closing the book always
flushes.
- In-process knownExistingDirs Set caches verified directories per app
session. createDir adds, removeDir (incl. recursive) clears. Cold
start still does the original exists+createDir dance once per dir.
loveheaven
1c392de0fa