OrganonTweaks/Asset/js/keep-scroll.js

/*
 * OrganonTweaks -- keep the board's horizontal scroll position across refreshes AND full reloads.
 *
 * Two cases lose the horizontal scroll and snap the board back to the first column:
 *   1. AJAX rebuild -- dropping a card / periodic polling replaces #board-container
 *      (BoardDragAndDrop.refresh), and the new element starts at scrollLeft 0.
 *   2. Full page reload -- e.g. clicking the Done/Todo badge, which navigates and redirects back to
 *      the board; a fresh page starts at scrollLeft 0.
 *
 * We remember the last position and restore it: in-memory for the AJAX rebuild (observing the STABLE
 * parent, since the container itself is replaced), and in sessionStorage (keyed per board) so it
 * survives a full reload as well.
 */
(function () {
    "use strict";

    function boardKey() {
        var m = location.href.match(/board\/(\d+)/) || location.href.match(/project_id=(\d+)/);
        return "organon-board-scroll-" + (m ? m[1] : location.pathname);
    }

    function readStored() {
        try {
            var v = window.sessionStorage.getItem(boardKey());
            return v ? parseInt(v, 10) : 0;
        } catch (e) {
            return 0;
        }
    }

    function writeStored(value) {
        try {
            window.sessionStorage.setItem(boardKey(), value);
        } catch (e) {
            // sessionStorage unavailable (private mode / disabled) -- degrade to AJAX-only restore.
        }
    }

    function init() {
        var container = document.getElementById("board-container");
        if (!container) {
            return;
        }

        var parent = container.parentNode;
        var stored = readStored();
        var lastScrollLeft = stored || container.scrollLeft;
        var lastContainer = container;

        // Restore across a full page reload (badge click, etc.), not only AJAX rebuilds.
        if (stored && container.scrollLeft !== stored) {
            container.scrollLeft = stored;
        }

        // Remember the position whenever the board is scrolled (capture: scroll does not bubble, and
        // this survives the container being replaced).
        document.addEventListener("scroll", function (e) {
            var c = document.getElementById("board-container");
            if (c && e.target === c) {
                lastScrollLeft = c.scrollLeft;
                writeStored(lastScrollLeft);
            }
        }, true);

        // When the board is rebuilt, #board-container becomes a new element at scrollLeft 0; restore
        // the remembered position before the browser paints it.
        if (window.MutationObserver) {
            new MutationObserver(function () {
                var c = document.getElementById("board-container");
                if (c && c !== lastContainer) {
                    lastContainer = c;
                    if (c.scrollLeft !== lastScrollLeft) {
                        c.scrollLeft = lastScrollLeft;
                    }
                }
            }).observe(parent, { childList: true });
        }
    }

    if (document.readyState === "loading") {
        document.addEventListener("DOMContentLoaded", init);
    } else {
        init();
    }
})();