Touch Scroll Speed Multiplier

Increases the touchscreen scroll speed in most websites.
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// ==UserScript==
// @name         Touch Scroll Speed Multiplier
// @namespace    http://tampermonkey.net/
// @version      1.4
// @license      MIT
// @description  Increases the touchscreen scroll speed in most websites.
// @match        *://*/*
// @grant        none
// ==/UserScript==

(function () {
    'use strict';

    const MULTIPLIER = 2.5;
    const START_THRESHOLD = 6;

    const FAIL_RATIO = 0.35;      // poniżej tego uznajemy, że scroll nie działa sensownie
    const FAIL_STREAK_LIMIT = 2;  // po tylu porażkach wyłączamy skrypt na stronie
    const STORAGE_KEY = 'tssm_disabled:' + location.hostname;

    let lastY = null;
    let isHandling = false;
    let movedEnough = false;

    let pageDisabled = false;
    let failureStreak = 0;
    let probeDone = false;

    try {
        if (sessionStorage.getItem(STORAGE_KEY) === '1') {
            pageDisabled = true;
        }
    } catch (_) {}

    function disableForThisPage() {
        pageDisabled = true;
        try {
            sessionStorage.setItem(STORAGE_KEY, '1');
        } catch (_) {}
    }

    function getRoot() {
        return document.scrollingElement || document.documentElement;
    }

    function isScrollable(el) {
        if (!(el instanceof Element)) return false;

        const style = getComputedStyle(el);
        const overflowY = style.overflowY;

        return /(auto|scroll|overlay)/.test(overflowY) && el.scrollHeight > el.clientHeight + 1;
    }

    function canScroll(el, delta) {
        const maxScrollTop = el.scrollHeight - el.clientHeight;
        if (maxScrollTop <= 0) return false;

        if (delta > 0) return el.scrollTop < maxScrollTop - 1;
        if (delta < 0) return el.scrollTop > 1;

        return false;
    }

    function getParent(el) {
        if (!el) return null;
        return el.parentElement || (el.getRootNode && el.getRootNode().host) || null;
    }

    function findScrollTarget(x, y, delta) {
        let el = document.elementFromPoint(x, y);

        while (el && el !== document.body && el !== document.documentElement) {
            if (isScrollable(el) && canScroll(el, delta)) {
                return el;
            }
            el = getParent(el);
        }

        const root = getRoot();
        return canScroll(root, delta) ? root : null;
    }

    function getScrollPos(target) {
        const root = getRoot();
        if (target === root) {
            return window.scrollY || window.pageYOffset || 0;
        }
        return target.scrollTop;
    }

    function probeRootScrollOnce() {
        if (probeDone || pageDisabled) return;
        probeDone = true;

        const root = getRoot();
        const maxScrollTop = root.scrollHeight - root.clientHeight;

        if (root.scrollHeight <= root.clientHeight + 5) return;

        const before = getScrollPos(root);
        const step = before < maxScrollTop - 1 ? 1 : -1;
        const testValue = before + step;

        root.scrollTop = testValue;

        const after = getScrollPos(root);
        const works = Math.abs(after - testValue) < 0.5;

        root.scrollTop = before;

        if (!works) {
            disableForThisPage();
        }
    }

    function recordScrollResult(target, before, amount) {
        requestAnimationFrame(() => {
            if (pageDisabled) return;

            const after = getScrollPos(target);
            const moved = Math.abs(after - before);
            const expected = Math.max(1, Math.abs(amount) * FAIL_RATIO);

            if (moved < expected) {
                failureStreak++;
                if (failureStreak >= FAIL_STREAK_LIMIT) {
                    disableForThisPage();
                }
            } else {
                failureStreak = 0;
            }
        });
    }

    function scrollTarget(target, amount) {
        const before = getScrollPos(target);
        const root = getRoot();

        if (target === root) {
            window.scrollBy(0, amount);
        } else if (typeof target.scrollBy === 'function') {
            target.scrollBy(0, amount);
        } else {
            target.scrollTop = before + amount;
        }

        recordScrollResult(target, before, amount);
    }

    function onTouchStart(e) {
        if (pageDisabled) return;

        probeRootScrollOnce();

        if (e.touches && e.touches.length === 1) {
            isHandling = true;
            movedEnough = false;
            lastY = e.touches[0].clientY;
        } else {
            isHandling = false;
            lastY = null;
        }
    }

    function onTouchMove(e) {
        if (pageDisabled) return;

        if (!isHandling || !e.touches || e.touches.length !== 1) {
            isHandling = false;
            movedEnough = false;
            return;
        }

        const touch = e.touches[0];
        const y = touch.clientY;
        const delta = lastY - y;
        lastY = y;

        if (!movedEnough && Math.abs(delta) < START_THRESHOLD) {
            return;
        }

        movedEnough = true;

        const target = findScrollTarget(touch.clientX, touch.clientY, delta);
        if (!target) return;

        e.preventDefault();
        scrollTarget(target, delta * MULTIPLIER);
    }

    function onTouchEnd() {
        isHandling = false;
        movedEnough = false;
        lastY = null;
    }

    document.addEventListener('touchstart', onTouchStart, { passive: true });
    document.addEventListener('touchmove', onTouchMove, { passive: false });
    document.addEventListener('touchend', onTouchEnd, { passive: true });
    document.addEventListener('touchcancel', onTouchEnd, { passive: true });
})();