Steps to Reproduce

Put a breakpoint in
org.zkoss.zk.ui.http.WpdExtendlet.Content.parse(RequestContext) (line 627)
at if (_cnt instanceof SourceInfo) in the following:

private Object parse(RequestContext reqctx) throws ServletException, IOException {
			if (_cnt instanceof SourceInfo)
				try {
					_cnt = ((SourceInfo) _cnt).parse(reqctx);
				} catch (IOException ex) {
					throw ex;
				} catch (ServletException ex) {
					throw ex;
				} catch (Exception ex) {
					throw UiException.Aide.wrap(ex);
				}
			return _cnt;
		}

Open a first ZK page on this server (Thread 1)
ON THREAD 1 - Resume until _cnt value is a SourceInfo object
ON THREAD 1 - Once _cnt is a SourceInfo instance, step inside the if statement, and stop before executing "_cnt = ((SourceInfo) _cnt).parse(reqctx);"
Open a 2nd ZK page in a difference tab (Thread 2)
ON THREAD 2 - resume the debugger until _cnt is NOT an instance of SourceInfo
ON THREAD 1 - resume the thread

Current Result

Exception thrown:

org.apache.catalina.core.StandardWrapperValve.invoke Servlet.service() for servlet [auEngine] in context with path [/Gui] threw exception
 java.lang.ClassCastException: org.zkoss.zk.ui.http.WpdExtendlet$ByteContent cannot be cast to org.zkoss.zk.ui.http.WpdExtendlet$SourceInfo
at org.zkoss.zk.ui.http.WpdExtendlet$Content.parse(WpdExtendlet.java:629)

And the page request returns unavailable since this happen during the filter chain

Expected Result

Pages should be displayed

Debug Info

Root Cause

Caused by static inner class Content field _cnt, accessed from multiple threads at the same time

Workaround

Attached replacement class in archive 20170508-WPDExtendletOverride, to add to the classPath while preserving it's package location.
Note: Performance may be affected by this workaround