// Roughly preserve reading position when entering the editor.
//
// While reading a page we record how far the center column is scrolled (as a
// fraction of its scrollable height, keyed by path). When the same path opens
// in the editor we restore that fraction, so you land near where you were
// reading instead of at the top. It is fraction-based — the rendered page and
// the raw-markdown editor have different heights — so the match is deliberately
// approximate. This only moves the viewport; the caret default (end of doc) is
// unchanged, so a click still decides where you actually edit.
(function () {
    var center = document.querySelector('.center');
    if (!center) return;

    var key = 'scrollsync:' + location.pathname;
    // Only restore a freshly-recorded position: a stale entry from earlier in
    // the tab session would jump a direct ?edit open to the wrong place.
    var MAX_AGE_MS = 5 * 60 * 1000;
    var params = new URLSearchParams(location.search);
    var inEditor = params.has('edit');
    // Section / insert edits load only a slice of the document into the editor,
    // so a whole-page reading fraction does not map onto them — restore only for
    // a full-page ?edit.
    var fullPageEdit = inEditor && !params.has('section') && !params.has('insert_before');

    function maxScroll() { return Math.max(0, center.scrollHeight - center.clientHeight); }

    if (inEditor) {
        if (!fullPageEdit) return;
        var raw = sessionStorage.getItem(key);
        if (!raw) return;
        var data;
        try { data = JSON.parse(raw); } catch (e) { return; }
        if (!data || typeof data.f !== 'number' || Date.now() - (data.t || 0) > MAX_AGE_MS) return;
        // Wait for CodeMirror to mount and the column to reach full height, then
        // map the fraction onto it. A double rAF after load lets CM's initial
        // layout and focus scroll settle so ours wins.
        window.addEventListener('load', function () {
            requestAnimationFrame(function () {
                requestAnimationFrame(function () {
                    center.scrollTop = data.f * maxScroll();
                });
            });
        });
        return;
    }

    // Reading mode: keep the latest scroll fraction recorded (throttled to one
    // write per frame) so it is ready the moment the editor opens.
    var pending = false;
    center.addEventListener('scroll', function () {
        if (pending) return;
        pending = true;
        requestAnimationFrame(function () {
            pending = false;
            var m = maxScroll();
            try {
                sessionStorage.setItem(key, JSON.stringify({ f: m > 0 ? center.scrollTop / m : 0, t: Date.now() }));
            } catch (e) {}
        });
    }, { passive: true });
})();