Hi FlashinnaPano,
I have played around your issue... and as you say, it seems that, setting the combobox visibility to false while the drop-down menu is displayed, makes the button part of the combobox to be hidden but not the drop-down menu...
Perhaps Klaus can improve the combobox plugin to avoid this

....
A way to avoid this is to set to false the capture attribute of your close button that call your hide action... this way, the click action on your close button pass also to the pano and closes the combobox drop-down menu ...
I have tried this way and it worked...
Hope this can help.
SAlut.