The Photos "moments" grid rendered every tile into the DOM, so a 20k-photo
library mounted ~140k nodes / 20k <img> elements, held ~196MB JS heap, took
~3.5s to first paint and scrolled at ~6fps. It also ran the justified-layout
maths inside the template (recomputed on every reactive change) and generated
client-side video thumbnails for every off-screen video, not just visible ones.
Introduce `VirtualRows` — a variable-height, section-aware sibling of
`VirtualList` — and flatten the grouped timeline into one list of fixed-height
rows (a date header or a strip of explicitly-sized tiles) shared by both the
square and justified layouts. Only the rows near the viewport are mounted; a
prefix-sum offset table + binary search find the visible band, and a spacer
reserves the full height so the sticky header and load-more sentinel are
unchanged. The justified packing now runs once per groups/width/layout change
in a $derived, not per render.
To avoid duplicating the scroll-tracking logic across the two windowing
components, extract it into a `useVirtualWindow` composable (scroll-ancestor
detection + rAF-throttled aboveBy/viewportH signals); `VirtualList` is
refactored onto it with identical measured numbers.
Measured in headless Chromium (1280x900), synthetic photos, before/after:
SQUARE | mount→tiles | DOM nodes | <img> | JS heap | scroll frame
------------+-------------+-----------+-------+---------+-------------
2000 | 416→94 ms | 14k→629 |2000→96| 21→5 MB | 29→29 ms
5000 | 916→114 ms | 35k→629 |5000→96| 50→9 MB | 62→26 ms
20000 | 3455→220 ms | 140k→629 |20k→96 |196→29 MB|152→33 ms
JUSTIFIED 20000: mount 245 ms · DOM 315 · <img> 44 · heap 33 MB · ~60fps
Rendered DOM, mounted <img> count and heap are now flat (O(visible)) regardless
of library size; mount is ~16x faster and scroll jank drops from 413 to ≤24
frames. Off-screen video-thumbnail generation no longer fires for non-visible
tiles. Correctness verified by probing a deep scroll in both layouts (tiles
land within the viewport band; square cells equal-width, justified rows
aspect-preserving).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M8Vb9QHmLZnEMzHz7MrFy6
Claude
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