Both advance-measurement paths measured a line by re-measuring a growing
prefix per grapheme, so one n-grapheme line shaped n^2/2 characters. The
web canvas measurer and the model fallback each carried a copy, and the
native Skia branch had the same shape via getRectsForRange(0, end) per
grapheme. Nothing caps line length client-side, so one long generated
line in a diff pinned the renderer main thread for minutes and the app
read as frozen while compositor animations kept running.
Advance measurement now has one owner in text-measurement.ts: 64-grapheme
chunks measured against an accumulated base, with chunk boundaries snapped
to shaping-safe seams so ligatures and contextual scripts keep their exact
prefix-measured widths (the xfi/Arabic contract in model.test.ts is
unchanged). A 1.2M-char line drops from a ~331-minute projection to under
a second; measurement-cost.test.ts pins the linear shaped-character
budget so the quadratic cannot come back.
Fast Refresh preserves refs while rerunning effects, so canceled paint requests must release their retained IDs or unseen diff regions stop rendering. Paint the final expanded body’s reserved boundary pixel because no following header owns that seam.
Reuse measured diff geometry when review controls open, keeping the interaction responsive on large changes. Preserve complete tab-track snapshots while exact content widths settle so new tabs and retained workspaces do not flash or truncate.
Keep diff headers and painted content on the same scroll clock while preserving native scrolling and independent per-file horizontal offsets.
Complete review, selection, typography, tree, and palette parity without returning line rendering to React.
Mohamed Boudra