test(studio): gate timeline viewport performance in Chromium#2697
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This was referenced Jul 21, 2026
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What
Add a reproducible Chromium performance gate for the timeline viewport contract.
Why
Unit tests can prove geometry and mount counts, but they cannot prove real-browser interaction latency, frame cadence, long-task behavior, reliable scroll width, or heap return at 50,000 elements.
How
This is D2 of the independent Family D draft Graphite stack. Its review base is the immutable, PR-less Family C baseline; Families A-C do not need to merge for this PR to be reviewed. It supersedes legacy PR #2633, which remains open for audit until landing.
Test plan
The unchanged strict gate passed twice consecutively on Chrome 150 with 50,000 elements and 1,000 tracks: 4/5 measured runs passed each sample, heap returned to baseline, direct scrolling was approved, and rendering stayed at 9 rows / 207 clips / 887 timeline descendants. The exact Family D tip also passed the full Studio and Studio Server validation matrix.