Reproducing step:

1. Use the following zul.

<zk>
<window id="win1" title="win1" mode="overlapped"  width="100px" left="50px" visible="false">
</window>
<window id="win2" mode="overlapped"  width="100px" left="150px" title="win2">
</window>
<separator height="400px"/>
<button label="show" onClick="win1.visible=true;win2.visible=true"/>
<button label="hide" onClick="win1.visible=false;win2.visible=false"/>
</zk>

win1 is an invisible window whose "left" attribute is set.
win2 is a visible window whose "left" attribute is set.

2. click "show" button.

a reproducing case in fiddle:
http://zkfiddle.org/sample/3qp9rgb/2-test-overlapped-window

Expected Result

"win1" is 50px left to screen's left border.

Actual Result

"win1" is at screen's left border.

Extra Test Result:

You can set "top", and you'll also get the same result. The invisible window's position doesn't change.
If we set "left" and "top" attributes at them same time, the invisible window location will be as expected.