The composer's `cursorLayout` (in `ui-tui/src/lib/inputMetrics.ts`) used a
hand-rolled word-wrap algorithm to decide where `useDeclaredCursor`
should park the hardware cursor. But Ink's `<Text wrap="wrap">` renders
the same text via `wrap-ansi`. The two algorithms disagreed on common
real-world inputs — `"branch investigate"` at cols=20, `"hello world"`
at cols=8, exact-fill strings like `"abcdefgh"` at cols=8 — so the
hardware cursor parked several cells past where Ink actually rendered
the last character. Users saw a multi-cell blank gap between their
last-typed letter and the cursor block, especially on narrow terminals
(the Cursor IDE built-in terminal was the worst offender).
Three previous PRs (#26717, #25860, #22197) chased fast-echo
displayCursor/cursorDeclaration drift and in-band-vs-native cursor
heuristics. None of them touched the underlying wrap-algorithm
mismatch, which is why the bug kept resurfacing.
Fix: source cursorLayout's line breaks from wrap-ansi directly. Walk
its emitted string char-by-char, tracking original-string offsets, push
a VisualLine at each '\n'. Also drop the buggy `column >= w` overflow
rule in cursorLayout — that's what pushed exact-fill text onto a
phantom next row.
canFastBackspaceShape now detects the wrap boundary in BOTH coordinate
conventions (column === 0 OR column >= columns), since exact-fill now
reports as (0, columns) instead of the previous (1, 0). The physical
state is identical — the terminal auto-wraps at column N either way —
but the layout function reports the position more honestly.
Tests:
- ui-tui/src/__tests__/textInputWrap.test.ts: 3 tests that pinned the
BUGGY behavior were updated to assert wrap-ansi parity (the real
invariant). Added a typing-prefix invariant: cursorLayout must agree
with wrap-ansi at every character of a long input.
- ui-tui/src/__tests__/cursorDriftRegression.test.ts: new file. Walks
the user-reported bug message char-by-char at 7 widths and asserts
agreement with wrap-ansi at every prefix.
Verification:
- 791/791 vitest tests pass.
- 84/84 tui-gateway pytest tests pass via scripts/run_tests.sh.
- PTY repro (typing into a real `hermes --tui` PTY at cols=50/55/60):
cursor lands exactly 1 cell past the last typed char in every case
the bug previously drifted.
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