WHAT CHANGED

In 8.0, ui/js/common.js:114 defines the xor helper with Object.defineProperty:

    Object.defineProperty(Array.prototype, 'xor',

{         value: function(arr) \{ ... }

,
        enumerable: false
    });

Attributes that are not named default to false, so the property is neither
writable nor configurable for the lifetime of the page. In 7.0 the same line is
a plain assignment (measured in the official image
zabbix/zabbix-web-nginx-pgsql:alpine-7.0), which leaves it writable and
configurable. 7.4 behaves like 7.0.

WHY IT BREAKS THIRD-PARTY FRONTEND MODULES

Assignment goes through [[Set]], and [[Set]] may not shadow a non-writable
inherited property in strict mode. Any library that builds a prototype from
Array.prototype and then uses Object.assign on it now throws.

Minimal reproduction, browser console on any 8.0 page - no module required:

    const proto = Object.create(Array.prototype);
    Object.assign(proto, { xor: function ()

{ return 'mine'; }

});

    // 7.0 / 7.4: ok
    // 8.0:       TypeError: Cannot assign to read only property 'xor' of object

CONCRETE IMPACT

Cytoscape.js does exactly this: it creates its collection prototype with
Object.create(Array.prototype) so collections behave array-like, then
Object.assigns its own xor (symmetric difference) onto it. On 8.0 the library
aborts while loading, window.cytoscape stays undefined, and the page that uses
it renders empty - with no error anywhere in the module's own code, because the
module code never runs. That makes it hard to trace: the failure surfaces in a
third-party bundle, not where the cause is.

Two independent installati