qmk_firmware/docs/feature_space_cadet_shift.md

Space Cadet Shift: The Future, Built In

Steve Losh described the Space Cadet Shift quite well. Essentially, when you tap Left Shift on its own, you get an opening parenthesis; tap Right Shift on its own and you get the closing one. When held, the Shift keys function as normal. Yes, it's as cool as it sounds.

Usage

Replace the Left Shift key in your keymap with KC_LSPO (Left Shift, Parenthesis Open), and Right Shift with KC_RSPC (Right Shift, Parenthesis Close).

Keycodes

Keycode	Description
KC_LSPO	Left Shift when held, ( when tapped
KC_RSPC	Right Shift when held, ) when tapped

Caveats

Space Cadet's functionality can conflict with the default Command functionality when both Shift keys are held at the same time. Make sure that Command is disabled in your rules.mk with:

COMMAND_ENABLE = no

Configuration

By default Space Cadet assumes a US ANSI layout, but if your layout uses different keys for parentheses, you can redefine them in your config.h. You can also disable the rollover, allowing you to use the opposite Shift key to cancel the Space Cadet state in the event of an erroneous press, instead of emitting a pair of parentheses when the keys are released. Also, by default, the Space Cadet applies modifiers LSPO_MOD and RSPC_MOD to keys defined by LSPO_KEY and RSPC_KEY. You can override this behavior by redefining those variables in your config.h. You can also prevent the Space Cadet to apply a modifier by defining DISABLE_SPACE_CADET_MODIFIER in your config.h.

Define	Default	Description
LSPO_KEY	KC_9	The keycode to send when Left Shift is tapped
RSPC_KEY	KC_0	The keycode to send when Right Shift is tapped
LSPO_MOD	KC_LSFT	The keycode to send when Left Shift is tapped
RSPC_MOD	KC_RSFT	The keycode to send when Right Shift is tapped
DISABLE_SPACE_CADET_ROLLOVER	Not defined	If defined, use the opposite Shift key to cancel Space Cadet
DISABLE_SPACE_CADET_MODIFIER	Not defined	If defined, prevent the Space Cadet to apply a modifier to LSPO_KEY and RSPC_KEY