Steps to Reproduce

Run fiddle

https://zkfiddle.org/sample/3r1hj1u/1-setVisible-doesn-t-trigger-parent-size-event

click 1st button

Current Result

test2 is hidden

parent doesn't resize, test3 doesn't resize

updated to feature:
see the doc below
https://www.zkoss.org/javadoc/latest/zk/org/zkoss/zk/ui/util/Clients.html#resize-org.zkoss.zk.ui.Component-

the flex=min part won't resize is expected.
Not only setVisible, any action that will cause the size changed need to call Clients.resize(parent) by them self.

Expected Result

ancestors should resize.

components synced to anscestors (such as flexing siblings) should resize.

updated to feature:
ZK being the UI framework, it feels wrong to ask the user to trigger these resizing themselves.
The whole point of flex is that the end-user doesn't need to control the UI manually.
Try to find a solution that user doesn't need to trigger these resizing themselves without performance impact.

Debug Information

Workaround

<script><![CDATA[ 
    zk.afterLoad("zul.wgt", function () {
        var _xDiv = {};
        zk.override(zul.wgt.Div.prototype, _xDiv, {
            setVisible: function() {
                var result = _xDiv.setVisible.apply(this, arguments);
                zUtl.fireSized(this.parent);
                return result;
            }
         });
    });
]]></script>