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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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