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A CDP trace of one sash drag settled what the render counters could not. I had assumed the remaining cost was layout/paint; it was not: script 6770ms | style 1866ms | layout 71ms Layout was never the problem. The top attributable callsite in our own code was use-resize-observer.ts at 977ms. Counting the callbacks named the mechanism exactly: 8,620 ResizeObserver instances constructed, and during a 40-move drag 2,600 callbacks each carrying exactly ONE entry — 65 separate callbacks per pointermove. Every consumer owned a private observer, so N elements resizing under a common ancestor meant N trips through the observer machinery instead of one batched delivery. With five mounted tiles that is ~100 user bubbles, each with its own observer, all woken by a width change. One shared observer with a WeakMap of target -> handlers. Callers keep their exact contract: a handler observing several elements is still invoked once with all of its entries, and unobserve happens when the last handler for an element goes away. Verified by trace, before -> after: use-resize-observer 977ms -> 42.5ms (-96%) style recalc 1866ms -> 1145ms (-39%) total script 6770ms -> 3929ms (-42%) Callback count 2,600 -> 43: one delivery per frame instead of 65. Adds the two probes that found it. diag-drag-trace.mjs takes a real timeline trace and prints the style/layout/script split plus the top script callsites — that split is what disproved the layout theory. diag-ro-storm.mjs counts RO callbacks vs entries, which is what distinguished 'a few expensive calls' from 'very many cheap ones'. |
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| .. | ||
| perf | ||
| .gitignore | ||
| after-pack.mjs | ||
| assert-dist-built.mjs | ||
| assert-dist-built.test.mjs | ||
| assert-root-install.mjs | ||
| before-build.mjs | ||
| before-pack.mjs | ||
| before-pack.test.mjs | ||
| bundle-electron-main.mjs | ||
| click-session.mjs | ||
| dev-mock.mjs | ||
| dev-no-hmr.mjs | ||
| diag-drag-churn.mjs | ||
| diag-drag-trace.mjs | ||
| diag-jump.mjs | ||
| diag-ro-storm.mjs | ||
| diag-scroll-reset.mjs | ||
| eval.mjs | ||
| gen-share-codes.ts | ||
| notarize-artifact.mjs | ||
| notarize.mjs | ||
| patch-electron-builder-mac-binary.mjs | ||
| probe-model-picker.mjs | ||
| probe-renderer.mjs | ||
| probe-thread.mjs | ||
| profile-model-picker.mjs | ||
| profile-typing-lag.md | ||
| rebuild-native.mjs | ||
| reload-renderer.mjs | ||
| reload.mjs | ||
| run-electron-builder.mjs | ||
| set-exe-identity.mjs | ||
| stage-native-deps.mjs | ||
| stage-native-deps.test.mjs | ||
| test-desktop.mjs | ||
| utils.mjs | ||
| write-build-stamp.mjs | ||
| write-build-stamp.test.mjs | ||