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// Exercise the real input/coast handlers with controlled frame timing.
import assert from 'node:assert/strict';
import { runInNewContext } from 'node:vm';
const source = await Bun.file('src/app.js').text();
const start = source.indexOf('(() => {', source.indexOf('// The feed scrolls like a phone'));
const end = source.indexOf("\nwindow.addEventListener('load'", start);
assert(start > 0 && end > start);
function fixture() {
let now = 100, id = 0;
const handlers = new Map(), frames = new Map(), steps = [];
const window = { scrollY: 5000,
addEventListener: (name, fn) => handlers.set(name, fn),
scrollBy: ({ top }) => { steps.push(top); window.scrollY += top; },
};
const document = { body: {}, addEventListener() {}, removeEventListener() {} };
const target = { closest: (selector) => selector.includes('.feed-page') ? target : null,
scrollHeight: 100, clientHeight: 100, parentElement: document.body };
runInNewContext(source.slice(start, end), { window, document,
matchMedia: () => ({ matches: false }), performance: { now: () => now },
requestAnimationFrame: (fn) => { frames.set(++id, fn); return id; },
cancelAnimationFrame: (id) => frames.delete(id), setTimeout, clearTimeout,
});
return {
get y() { return window.scrollY; }, steps,
wheel(dir) { handlers.get('wheel')({ deltaY: dir * 100, wheelDeltaY: -dir * 120,
deltaMode: 0, deltaX: 0, target, timeStamp: now, preventDefault() {} }); },
advance(ms) { now += ms; },
frame(stamp = now) { const callbacks = [...frames.values()]; frames.clear(); for (const fn of callbacks) fn(stamp); },
};
}
const settle = (f, n = 70) => { for (let i = 0; i < n; i++) { f.advance(17); f.frame(); } };
for (const dir of [1, -1]) {
const f = fixture();
f.wheel(dir); f.advance(17); f.frame();
f.wheel(dir); f.advance(17); f.frame();
// A busy main thread has missed several frames when the reverse arrives.
f.advance(120);
const before = f.y;
f.wheel(-dir);
assert((f.y - before) * dir < 0, 'first reverse notch responds immediately');
assert(Math.abs(f.y - before) < 20, 'reverse starts with a small bounded step');
const reversed = f.y;
f.frame(0); // a queued animation frame can carry an older timestamp
assert.equal(f.y, reversed, 'old frame time cannot revive the previous direction');
f.advance(17); f.frame();
assert((f.y - reversed) * dir < 0, 'next frame continues in the new direction');
assert(Math.abs(f.y - reversed) < 20, 'new velocity is not applied to the old frame delay');
settle(f);
// The turn back is a plain notch from rest: its full travel, no flight the other way.
const back = (before - f.y) * dir;
assert(back > 280 && back < 330, 'a reverse notch against a coast travels one notch: ' + back);
// For a moment after a reversal notches don't wind the speed up: a
// reversal and a quick second notch travel about two notches, not a flight.
let calm;
{ f.wheel(-dir); f.advance(17); f.frame(); f.wheel(-dir); f.advance(120); f.frame(); // coasting again, the other way
const y = f.y; f.wheel(dir); f.advance(60); f.frame(); f.wheel(dir); settle(f); calm = (f.y - y) * dir;
assert(calm > 540 && calm < 700, 'a reversal and a quick second notch: about two notches of travel: ' + calm); }
// From rest, well after, the wind-up is back: the same two quick notches go further.
{ f.advance(2500); const y = f.y; f.wheel(dir); f.advance(60); f.frame(); f.wheel(dir); settle(f); const s = (f.y - y) * dir; assert(s > calm + 40, 'two quick notches with the wind-up go further: ' + s + ' vs ' + calm); }
// A spin that keeps going after a reversal winds up again from its third
// notch: a reversal plus four quick notches goes well past five plain ones.
let spin;
{ f.wheel(-dir); f.advance(17); f.frame(); f.wheel(-dir); f.advance(120); f.frame();
const y = f.y; f.wheel(dir); for (let i = 0; i < 4; i++) { f.advance(60); f.frame(); f.wheel(dir); } settle(f); spin = (f.y - y) * dir;
assert(spin > 5 * 300 + 150, 'a spin on after a reversal winds up again: ' + spin); }
// ...and the same five quick notches from rest go about as far (the two calm ones cost little).
{ f.advance(2500); const y = f.y; f.wheel(dir); for (let i = 0; i < 4; i++) { f.advance(60); f.frame(); f.wheel(dir); } settle(f); const s = (f.y - y) * dir;
assert(s < spin * 1.25, 'five from rest vs five after a reversal: ' + s + ' vs ' + spin); settle(f, 200); }
// From rest a notch either way is a full push.
{ f.advance(2500); const y = f.y; f.wheel(dir); settle(f); const s = (f.y - y) * dir; assert(s > 280 && s < 330, 'from rest a notch either way is a full push: ' + s); }
// A turn back against a coast that only just died is a reversal too (caught, no wind-up).
{ f.wheel(dir); settle(f, 40); f.advance(100); const y = f.y; f.wheel(-dir); f.advance(60); f.frame(); f.wheel(-dir); settle(f); const s = (y - f.y) * dir; assert(s > 520 && s < 700, 'reversing right after the coast dies: no wind-up: ' + s); }
}
console.log('✓ immediate reversal both ways, stale frame timestamps, busy frame recovery, a reversal is a plain notch, two calm notches, then the wind-up is back');
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