Last week we upgraded Citrix, which for some reason broke the ability to alt-tab in fullscreen mode to send that command to the remote computer.
There are instructions in the connection window to set it so it goes to the remote, but those instructions didn’t work for me on windows 10 host going to a windows 7 remote box.
After going without it for a week, I got time this morning to google the fix, which is updating a registry key.
Computer\HKEY_CURRENT_USER\Software\Citrix\ICA Client\Engine\Lockdown Profiles\All Regions\Lockdown\Virtual Channels\Keyboard
Now when I log in to work, the windows, alt- keys and everything else works much better!