Menu list remains after dialog box closed in Lyx 2.1 on Linux

[Matthieu]

Hi

I recently upgraded to Lyx 2.1 in Linux, and noticed that sometimes, a 
menu list will remain open even if the dialog box has been closed in the 
meanwhile. And that open menu list will superimpose on other 
applications. A screenshot will probably make my point clearer:

https://dl.dropboxusercontent.com/u/6113358/Permanent/Lyx_2.png
https://dl.dropboxusercontent.com/u/6113358/Permanent/Lyx_Bug.png
https://dl.dropboxusercontent.com/u/6113358/Permanent/Lyx_menu_freeze.png

to close the menu list, one needs to go back to Lyx, go to the dialog 
box, and click again on the selected field. This happens often (see 
three screenshots), but not every time. Problem encountered on Lyx LyX 
Version 2.1.0 (Sunday, April 13, 2014) on Ubuntu 12.4

Kindly keep me on on cc for replies, as I am not subscribed to lyx-devel 
mailing list.

Thanks

Matthieu