Answers:
来自man xkeyboard-config：
更改布局的钥匙
┌───────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│Option Description │
├───────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│grp:switch Right Alt (while pressed) │
│grp:lswitch Left Alt (while pressed) │
│grp:lwin_switch Left Win (while pressed) │
│grp:rwin_switch Right Win (while pressed) │
│grp:win_switch Any Win key (while pressed) │
│grp:caps_switch Caps Lock (while pressed), Alt+Caps Lock does the original capslock action │
│grp:rctrl_switch Right Ctrl (while pressed) │
│grp:toggle Right Alt │
│grp:lalt_toggle Left Alt │
│grp:caps_toggle Caps Lock │
│grp:shift_caps_toggle Shift+Caps Lock │
│grp:shift_caps_switch Caps Lock (to first layout), Shift+Caps Lock (to last layout) │
│grp:win_menu_switch Left Win (to first layout), Right Win/Menu (to last layout) │
│grp:lctrl_rctrl_switch Left Ctrl (to first layout), Right Ctrl (to last layout) │
│grp:alt_caps_toggle Alt+Caps Lock │
│grp:shifts_toggle Both Shift keys together │
│grp:alts_toggle Both Alt keys together │
│grp:ctrls_toggle Both Ctrl keys together │
│grp:ctrl_shift_toggle Ctrl+Shift │
│grp:lctrl_lshift_toggle Left Ctrl+Left Shift │
│grp:rctrl_rshift_toggle Right Ctrl+Right Shift │
│grp:ctrl_alt_toggle Alt+Ctrl │
│grp:alt_shift_toggle Alt+Shift │
│grp:lalt_lshift_toggle Left Alt+Left Shift │
│grp:alt_space_toggle Alt+Space │
│grp:menu_toggle Menu │
│grp:lwin_toggle Left Win │
│grp:rwin_toggle Right Win │
│grp:lshift_toggle Left Shift │
│grp:rshift_toggle Right Shift │
│grp:lctrl_toggle Left Ctrl │
│grp:rctrl_toggle Right Ctrl │
│grp:sclk_toggle Scroll Lock │
│grp:lctrl_lwin_rctrl_menu LeftCtrl+LeftWin (to first layout), RightCtrl+Menu (to second layout) │
│ │
└───────────────────────────────────────────────────────────────────────────────────────────────────────────┘
man xkeyboard-config也显示grp:win_space_toggle Win+Space（xkeyboard-config 2.22）。