run the attached example using ZK CE or PE (the problem does not happen in ZK EE, assumption client ROD fixes this problem)

when causing an AU request (click the button)
-> the borderlayouts in the previously never opened tabpanels resize their west panel to 0px even if an explicit width is defined.

without causing an AU request (i.e. not clicking the button)
-> the borderlayouts show correctly, clicking the button later does not affect the size of the west panel anymore

debugging the JS code I found that line 164 of Borderlayout.js performs a visibility check, somehow it continues even though the borderlayout is hidden in an invisible tabpanel
https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/layout/Borderlayout.js#L164