The thumb button of the MX Master is not a button but a simulated keystroke. The issue being that I can't figure out which one it is since xev doesn't recognize it. The shortcuts panel tells me it's CTR + ALT + TAB but it's not doing what CTR + ALT + TAB should do, namely switch between panels. Instead what the thumb button does is going back and forth from the window I'm on to the last one displays, ad infinitum. Does anyone know what this function is?Edit (result from pressing two times the thumb button):Event: time 1486843409.135360, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e0Event: time 1486843409.135360, type 1 (EV_KEY), code 29 (KEY_LEFTCTRL), value 1Event: time 1486843409.135360, -------------- SYN_REPORT ------------Event: time 1486843409.143358, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e2Event: time 1486843409.143358, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 1Event: time 1486843409.143358, -------------- SYN_REPORT ------------Event: time 1486843409.151354, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002bEvent: time 1486843409.151354, type 1 (EV_KEY), code 15 (KEY_TAB), value 1Event: time 1486843409.151354, -------------- SYN_REPORT ------------Event: time 1486843409.159360, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002bEvent: time 1486843409.159360, type 1 (EV_KEY), code 15 (KEY_TAB), value 0Event: time 1486843409.159360, -------------- SYN_REPORT ------------Event: time 1486843409.167356, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e2Event: time 1486843409.167356, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 0Event: time 1486843409.167356, -------------- SYN_REPORT ------------Event: time 1486843409.175354, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e0Event: time 1486843409.175354, type 1 (EV_KEY), code 29 (KEY_LEFTCTRL), value 0Event: time 1486843409.175354, -------------- SYN_REPORT ------------Event: time 1486843410.633371, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e0Event: time 1486843410.633371, type 1 (EV_KEY), code 29 (KEY_LEFTCTRL), value 1Event: time 1486843410.633371, -------------- SYN_REPORT ------------Event: time 1486843410.641373, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e2Event: time 1486843410.641373, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 1Event: time 1486843410.641373, -------------- SYN_REPORT ------------Event: time 1486843410.649370, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002bEvent: time 1486843410.649370, type 1 (EV_KEY), code 15 (KEY_TAB), value 1Event: time 1486843410.649370, -------------- SYN_REPORT ------------Event: time 1486843410.657362, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002bEvent: time 1486843410.657362, type 1 (EV_KEY), code 15 (KEY_TAB), value 0Event: time 1486843410.657362, -------------- SYN_REPORT ------------Event: time 1486843410.665363, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e2Event: time 1486843410.665363, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 0Event: time 1486843410.665363, -------------- SYN_REPORT ------------Event: time 1486843410.673364, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e0Event: time 1486843410.673364, type 1 (EV_KEY), code 29 (KEY_LEFTCTRL), value 0 Edit II I managed to get the result from xev. Also from pressing two times:KeyPress event, serial 37, synthetic NO, window 0x6400001,      root 0xf5, subw 0x0, time 20443872, (-455,-283), root:(1367,722),      state 0x10, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,      XLookupString gives 0 bytes:       XmbLookupString gives 0 bytes:       XFilterEvent returns: FalseKeyPress event, serial 37, synthetic NO, window 0x6400001,      root 0xf5, subw 0x0, time 20443880, (-455,-283), root:(1367,722),      state 0x14, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,      XLookupString gives 0 bytes:       XmbLookupString gives 0 bytes:       XFilterEvent returns: FalseFocusOut event, serial 37, synthetic NO, window 0x6400001,      mode NotifyGrab, detail NotifyAncestorFocusOut event, serial 37, synthetic NO, window 0x6400001,      mode NotifyWhileGrabbed, detail NotifyNonlinearPropertyNotify event, serial 37, synthetic NO, window 0x6400001,      atom 0x16b (_NET_WM_STATE), time 20443950, state PropertyNewValueFocusIn event, serial 37, synthetic NO, window 0x6400001,      mode NotifyWhileGrabbed, detail NotifyNonlinearKeymapNotify event, serial 37, synthetic NO, window 0x0,      keys:  0   0   0   0   32  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   FocusIn event, serial 37, synthetic NO, window 0x6400001,      mode NotifyUngrab, detail NotifyNonlinearKeymapNotify event, serial 37, synthetic NO, window 0x0,      keys:  3   0   0   0   32  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   KeyRelease event, serial 37, synthetic NO, window 0x6400001,      root 0xf5, subw 0x0, time 20445256, (-455,-283), root:(1367,722),      state 0x14, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,      XLookupString gives 0 bytes:       XFilterEvent returns: FalsePropertyNotify event, serial 37, synthetic NO, window 0x6400001,      atom 0x16b (_NET_WM_STATE), time 20445283, state PropertyNewValue
Setting mouse MX Master thumb button on Mint 18.1
mouse