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