[chrismorgan]

Drop it in a user script with Greasemonkey or similar, and add a keypress event listener.

The bones of it:

  // ==UserScript==
  // @name     Personal jmapui tweaks
  // @version  1
  // @grant    none
  // ==/UserScript==

  addEventListener("keypress", event => {
    if (event.target …) { return; }
    if (event.key == 'x' && event.ctrlKey && !event.metaKey …) {
      window.eval('…');
    }
  });

You’ll want to do things like skip events targeted at form elements or within a contenteditable, perhaps unless you use a suitable modifier.

It may be helpful to flatten the modifiers and key into a single string, like "Ctrl-Meta-Shift-S", and match on that.

The security interactions between your user script code and document code are fiddly and troublesome, and debugging is distressingly limited. I find using window.eval(`…`) to be a good technique for executing arbitrary code in the security context of the page itself.