How to make the web version of Wunderlist resizable panel?

Question

css

I constantly use Wunderlist for work, gradually almost all personal projects moved to it. It is good for everyone, but for some reason in the web version it is impossible to resize the content panel on the right.
The essence of the request

How can I achieve
Locally in my Firefox and Chrome I made a simple CSS injection through Stylish:

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("wunderlist.com") {
 #detail {
   resize: horizontal;
   overflow: auto;
   max-width: 50%;
   width: 560px;
 }
}

The problem is that the resize handle is stuck in the lower right corner of the screen. Because of this, the panel can only be reduced from a given size, which is inconvenient. So for now I made it even simpler:

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("wunderlist.com") {
 #detail {
    width: 500px !important;
 }
}

What I Really Want
To take the resizable widget code from here (see the Left section) and make an extension for Firefox and Chrome that makes folder and content panes resizable.
Ready to discuss implementation for money.