I got sick of having to slowly scroll down discussions on Canvas, so made a userscript to mark all unread posts as read for me. Adds a button to the header on each discussion page.

Canvas-Mark-All-As-Read.user.js

// ==UserScript==
// @name         Canvas 'Mark All as Read'
// @namespace    com.plasticsquid.canvas.read
// @version      1.0
// @description  Adds a button to allow marking of all posts as read in a Canvas discussion
// @author       Cary Symes
// @match        https://*.instructure.com/courses/*/discussion_topics/*
// @grant        none
// ==/UserScript==


// DISCLAIMER
// I threw this together in an enraged fury, and so cannot guarantee that
// it is in any way, shape or form bug-free or even unlikely
// to cause the apocalypse. But hey, at least it works on my machine.
// Also any change to Canvas may break this at any time.

// Hosted at https://gist.github.com/CSymes/a287349bf732a8b018762e3370ddf153


function MarkAsRead() {
    // Click on all unread indicators (case insensitive)
    $('a.discussion-read-state-btn[title="Mark as Read" i]').click();
    // Will mark them as read
}

(function() {
    'use strict';

    // Header of the discussion page - already contains buttons
    var header = $('#discussion-toolbar .span9');

    // Let's add another
    var marker = $('<button/>', {
        text: 'Mark All As Read',
        id: 'MAAR',
        class: 'btn', // Style it like the other buttons
        click: MarkAsRead // Callback
    });
    marker.css('margin-left', '1em'); // Pad it apart from the other buttons

    header.append(marker);
})();