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.

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
DISABLE_SPACE_CADET_ROLLOVER	Not defined	If defined, use the opposite Shift key to cancel Space Cadet