I faced the same problem - I'm using online software that uses Alt+Mouse to rotate camera view and I was annoyed seeing Opera menu pop up every time I rotated camera. When I understood that in new Opera customizing keyboard isn't possible, I tried to make a custom extension that will just run the script everywhere and disable default Alt's action on every page, but that turned out harder than I thought. Then I found this Violent monkey Extension, that lets you execute your custom scripts everywhere or just on specific websites. Just add this script to the extension and you're good to go. // ==UserScript== // @name Disable Alt key // @include http://<specific site's domain>/* // @grant none // ==/UserScript== window.onkeydown = function (event) { if (event.keyCode === 18) { event.preventDefault(); } };