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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); },
};
}
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');
for (let i = 0; i < 100; i++) { f.advance(17); f.frame(); }
assert(Math.abs(f.y - before) > 280 && Math.abs(f.y - before) < 330, 'one notch retains its coast distance');
}
console.log('✓ immediate reversal both ways, stale frame timestamps, busy frame recovery, and coast distance');
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