Steps to Reproduce

hide the version information:

    <library-property>
        <name>org.zkoss.zk.ui.versionInfo.enabled</name>
        <value>false</value>
    </library-property>

load any zul page containing dynamically loaded widget packages.
e.g.:

<zk>
  <textbox/>
</zk>

observe the URL loading the JS package zul.inp.wpd

Current Result

independent of the used ZK version this URL remains the same, effectively disabling the cache breaking mechanism for js packages:

http://localhost:8080/zksupport85/zkau/web/js/zul.inp.wpd

Expected Result

a cache breaking part of the URL (obfuscating the actual version/built time)
http://localhost:8080/zksupport85/zkau/web/_zv2018082816/js/zul.inp.wpd

Debug Info

Root Cause

the version info is empty:
https://github.com/zkoss/zk/blob/v8.6.1/zk/src/org/zkoss/zk/ui/http/WpdExtendlet.java#L646

Workaround

specify cache breaking <javascript-module> versions in a lang-addon.xml
for all root packages loaded dynamically in the application (when upgrading the ZK version, also update the version string)

e.g.: for "zul.inp" it is sufficient to specify a module version for the root "zul"

<?xml version="1.0" encoding="UTF-8"?>
<language-addon>
	<addon-name>cache breaker</addon-name>
        <!-- workaround for https://tracker.zkoss.org/browse/ZK-4287 fixed since 8.6.2 -->
	<javascript-module name="zk" version="my-version-1"/>
	<javascript-module name="zul" version="my-version-1"/>
	...
</language-addon>

this is a manual workaround and the version attribute needs to be updated to change the urls between affected ZK versions, remove this after upgrading to 8.6.2 or higher