I want the panel to stay in the back of other windows instead of hiding. There is such an option in the panel settings but seems not to be working.
I managed to do that by editing /home/me/.config/openbox/lubuntu-rc.xml
Add these lines just before the last line:

<application name="panel">
<skip_taskbar>yes</skip_taskbar>
<layer>above</layer>
</application>
<application name="panel" type="dock">
<layer>below</layer>
</application>
</applications>

Then restart lubuntu