This stops auto-play of videos (vines, GIFs) on TweetDeck. You can still manually play videos by clicking them, though.

stop_tweetdeck_videos.user.js

// ==UserScript==
// @name          Auto-scroll in rabb.it chat
// @namespace     http://panzi.github.io/
// @description   Automaticall scroll the chat on rabb.it to the bottom.
// @include       https://www.rabb.it/*
// @include       http://www.rabb.it/*
// @include       https://rabb.it/*
// @include       http://rabb.it/*
// @version       1.0
// ==/UserScript==

"use strict";

var autoScrollObservers = [];

function installAutoScroll () {
	uninstallAutoScroll();

	var scrollAreas = document.querySelectorAll(".conversationMessageCollectionView");
	Array.prototype.forEach.call(scrollAreas, scrollArea => {
		var observer = new MutationObserver(() => {
			// delay this after the mutation so that we get the updated values
			setTimeout(scrollToBottom, 0);
			setTimeout(scrollToBottom, 300);
		});
		observer.observe(scrollArea, {childList: true, subtree: true, characterData: true});
		autoScrollObservers.push(observer);

		function scrollToBottom () {
			if (scrollArea.scrollHeight > scrollArea.offsetHeight &&
				scrollArea.scrollHeight <= (scrollArea.offsetHeight + scrollArea.scrollTop + 300)) {
				scrollArea.scrollTop = scrollArea.scrollHeight - scrollArea.offsetHeight;
			}
			else {
				console.log('no auto scroll', scrollArea.scrollTop, scrollArea.scrollHeight, scrollArea.offsetHeight);
			}
		}
	});
}


function uninstallAutoScroll () {
	autoScrollObservers.forEach(observer => observer.disconnect());
	autoScrollObservers = [];
}

installAutoScroll();