Ttgjyuk

I recently got the Logitech M720. Pressing the thumb button in xev gives the following:

KeyPress event, serial 36, synthetic NO, window 0x2c00001,
 root 0x270, subw 0x0, time 1922861, (169,43), root:(248,150),
 state 0x0, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
 XLookupString gives 0 bytes: 
 XmbLookupString gives 0 bytes: 
 XFilterEvent returns: False

KeyPress event, serial 36, synthetic NO, window 0x2c00001,
 root 0x270, subw 0x0, time 1922894, (169,43), root:(248,150),
 state 0x4, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
 XLookupString gives 0 bytes: 
 XmbLookupString gives 0 bytes: 
 XFilterEvent returns: False

LeaveNotify event, serial 36, synthetic NO, window 0x2c00001,
 root 0x270, subw 0x0, time 1922894, (169,43), root:(248,150),
 mode NotifyNormal, detail NotifyAncestor, same_screen YES,
 focus YES, state 12

EnterNotify event, serial 36, synthetic NO, window 0x2c00001,
 root 0x270, subw 0x0, time 1922920, (169,43), root:(248,150),
 mode NotifyNormal, detail NotifyAncestor, same_screen YES,
 focus YES, state 12

KeymapNotify event, serial 36, synthetic NO, window 0x0,
 keys: 112 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Trying to bind it in gnome keyboard shortcuts does not work either. There is no button entry in xev.

Using xev | grep keycode:

state 0x0, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
state 0x4, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,

In windows this button causes an alt tab, is it trying to do the same here?

I had the same issue before. I accidentally solved it after reinstalling Ubuntu 16.04 for other reasons.

In my case, it was imwheel restricting the use of thumb buttons. In my .imwheelrc, I have

".*"
None, Up, Up, 5
None, Down, Down, 5

It was meant to increase scroll speed, but also blocks thumb buttons (You do NOT have to use the same setting as mine in imwheelrc). To make them available again, in terminal,first do: killall imwheel then do imwheel -b "4 5".
It works by limiting the button grab to the specified buttons.

Note: I do **NOT** have `xautomation` installed. I would try uninstall xautomation if above steps doesn't work.

Why not to change the keyboard shortcut Ctrl+Alt+Tab to the one you want?