* perf(app): keep large diffs responsive
Render file headers on the canvas and bound expensive text shaping to the active viewport while preserving sticky, hover, and horizontal-scroll behavior. Let native scroll cancellation arbitrate header presses so finger scrolling cannot expand files.
* test(app): assert canvas diff header accessibility
* test(app): assert canvas diff header semantics
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.
Mohamed Boudra