X Keyboard Extension - Latched and Locked Modifiers

Latched and Locked Modifiers

XKB allows a modifier to be locked or latched, other than being in its regular state. Normally, a modifier is active exactly when it is pressed, like the Shift. However, a modifier may also be locked, like the Caps Lock modifier. In particular, when a modifier is locked, it remains active until it is explicitly deactivated. An intermediate condition between regular and locked is the latched state: When a modifier is latched, it remains active, but only until the next non-modifier key is pressed.

XKB allows a client application to explicitly latch or lock a modifier. Moreover, an application can bind a key press or release to a modifier state change. This way, a modifier may automatically become latched or locked whenever a key is pressed or released.