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libxkbcommon/doc/compat.md

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Compatibility

Relative to the XKB 1.0 specification implemented in current X servers, xkbcommon has removed support for some parts of the specification which introduced unnecessary complications. Many of these removals were in fact not implemented, or half-implemented at best, as well as being totally unused in the standard dataset.

Notable removals:

  • geometry support
    • there were very few geometry definitions available, and while xkbcommon was responsible for parsing this insanely complex format, it never actually did anything with it
    • hopefully someone will develop a companion library which supports keyboard geometries in a more useful format
  • KcCGST (keycodes/compat/geometry/symbols/types) API
    • use RMLVO instead; KcCGST is now an implementation detail
    • including pre-defined keymap files
  • XKM support
    • may come in an optional X11 support/compatibility library
  • around half of the interpret actions
    • pointer device, message and redirect actions in particular
  • non-virtual modifiers
    • core and virtual modifiers have been collapsed into the same namespace, with a 'significant' flag that largely parallels the core/virtual split
  • radio groups
    • completely unused in current keymaps, never fully implemented
  • overlays
    • almost completely unused in current keymaps
  • key behaviors
    • used to implement radio groups and overlays, and to deal with things like keys that physically lock; unused in current keymaps
  • indicator behaviours such as LED-controls-key
    • the only supported LED behaviour is key-controls-LED; again this was never really used in current keymaps

On the other hand, some features and extensions were added.

Notable additions:

  • 32-bit keycodes
  • extended number of modifiers (planned)
  • extended number of groups (planned)
  • multiple keysyms per level
    • such levels are ignored by x11/xkbcomp.
  • key names (e.g. <AE11>) can be longer than 4 characters.

Compose support

Relative to the standard implementation in libX11 (described in the Compose(5) man-page), some features are not supported:

  • the (! MODIFIER) syntax
    • parsed correctly but ignored.
  • using modifier keysyms in Compose sequences
  • several interactions with Braille keysyms