Alt+Tab doesn't work when Citrix is in full screen

Finally it's now working. I'm actually accessing remote desktop through web browser Google Chrome on Ubuntu 12.04.

I changed all the setting in Citrix receiver, but didn't realize to change the setting on web page. That is, after typing username/password for the remote URI in the browser, there is a tab called setting (before click Remote Desktop Connect on AmericasXA...).

I was able to change the window size to full screen from Seamless. That's it, Alt+Tab and other keys started working fine on the remote desktop.

Edit the file /etc/icaclient/config/All_Regions.ini, and set the value TransparentKeyPassthrough=Remote.

Example All_Regions.ini file:

...
...
; --- on my system, the setting is on line 601
; These settings control the keyboard settings
;
[Virtual Channels\Keyboard]
TransparentKeyPassthrough=Remote
...

...

This answer was the key to solving the problem on my system.