Opening the Library with a large grouped library was slow because each
folder's 2x2 cover mosaic was recomposed from scratch on every paint (up
to 4 MuPDF cover decodes plus scales per cell). On a 685-book library this
dominated the synchronous open path (254ms of 300ms) and ran again on the
post-sync refresh. Navigation felt fast only because drilling into a group
shows single covers, not mosaics.
Mirror cloud_covers' async pattern in group_covers:
- Cache the composed master bb per group, keyed by a signature that flips
when the child set or any child's cover availability changes; serve
cheap copies on a hit. Cache a nil result too, so a group whose covers
are not ready yet keeps its placeholder without recomposing on every
refresh; a later cover download flips the signature and recomposes once.
- Compose off the first-paint path: a miss enqueues a background job (one
mosaic per UI tick) and returns nil so the cell paints its FakeCover
placeholder immediately; finished mosaics coalesce into one refresh.
- Free cached masters when the Library closes.
Measured synchronous open path drops from 300ms to 151ms on a 685-book
library; mosaic compositing moves off the blocking paint and fills in
progressively. Adds open-path timing logs to librarywidget and
localscanner for on-device diagnosis of future large-library reports.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Huang Xin
f805477091