One thing to try would be to add a bogus query variable to the manager url
current url ?current args&x=123
That should force the browser to reload the JS (which may not have been reloaded after you turned off compress_js).
I doubt if it will help, but the this.mask.addClass line is in this file:
manager\assets\modext\widgets\core\modx.window.js
I think it has to do with beautifying some animations in the Manager.
It's in the onShow function, so you could try just commenting out that line (60) or the whole inside of the function.