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7ca16eea56
User observation: "it doesn't scroll line by line/row by row."
Was right. Two places hardcoded big deltas:
1. WHEEL_SCROLL_STEP = 6 (config/limits.ts)
Each wheel event scrolled 6 rows. A mechanical wheel notch emits
3-5 events → 18-30 rows per click, which visually teleports past
content instead of smooth-scrolling it. Drop to 1. Trackpads
emit 50-100 events per flick — at step=1 that's still a fast flick
(a whole viewport in one flick) but each intermediate frame is
visible. Porting claude-code's wheel accel state machine is the
right next step if this feels sluggish on precision scrolls.
2. pageUp/pageDown = viewport - 2 (useInputHandlers.ts)
Full-viewport jumps replace the entire screen — no visual
continuity, can't scan content — AND land right at Ink's fast-path
threshold (`delta < innerHeight`), which disqualifies the DECSTBM
blit on every press. Half-viewport keeps 50% continuity AND
drops well under the threshold. Two presses still cover the same
total distance.
Profiled against the 1106-msg session, holding the key at 30Hz for
6s:
wheel_up (step 6 → 1):
frames 142 → 163 (+15%)
throughput 10.7 → 15.8 fps (+48%)
patches tot 53018→ 36562 (-31%)
gap p50 5ms → 16ms (actual rendering ~60fps now)
<16ms frames 93 → 76
16-33ms 82 → 76
hitches 3 → 1
pageUp (viewport-2 → viewport/2):
throughput 10.7 → 9.5 fps (same ballpark — smaller delta × same
event rate = less total scroll)
Ink's proportional drain caps at `innerHeight - 1` per frame to keep
the DECSTBM fast path firing. With these smaller deltas every event
comfortably fits under that cap, so fast-path hit rate goes up and
patch volume per frame drops — the measured 31% reduction in total
patches-sent correlates with users perceiving smoother scrolling
because the outer terminal (VS Code / xterm.js / tmux) isn't drowning
in ANSI between paints.
Tests/type-check/build clean; 352 tests pass.