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UCW keyboard layout on XWayland
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:slug: ucw-keymap-on-wayland
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:date: 2023-12-31 18:23
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:tags: xkb, linux, xwayland
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:category: random
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:keywords: xkb, xorg, wayland, hack
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:lang: en
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:translation: false
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:status: draft
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This is a story/writeup of how I debugged UCW layout on XWayland. You may learn
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here how keyboard layouts work on Linux.
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Background: The UCW layout
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==========================
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The UCW keyboard layout is one of the interesting methods of typing Czech
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letters on a rather American keyboard. The main idea is having a classic US
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(QWERTY) layout, but without the CapsLock, which serves as a key to add
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diacritics, so that e.g. Caps+s creates "š". It is naturally possible to
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combine Caps with Shift to create uppercase letters, and because not all
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letters can have diacritics in Czech (e.g. there is no "ǩ"), it also manages to
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cover all the German and Slovak letters. (Sometimes, there are multiple
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diacritics for a single letter, then the additional diacritics are nearby:
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Caps+e is "é", Caps+w is "ě".) The CapsLock can still be pressed by using
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Alt+Caps.
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The layout has some nice features like avoiding deficiencies with the Czech
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layout (diacritics on number row are too far, both round parentheses are on the
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same key, other parentheses only with AltGr at random places, no asterisk, for
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some reason we have "§" though, …). Another nice feature is that it is rather
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interoperable: I am able to type on any computer with the "most standard"
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layout and when writing in Czech I can just not press the CapsLock key and only
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lose my diacritics, computers can have this layout system-wide even when
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foreign people use it,…
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I would not say many people use the layout but at least several my friends do
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and I have come across several random people on the Internet who do also. More
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on this below :-)
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The important part is how the layout is set up. Fortunately, it is contained in
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the xkeyboard-config database, so the following command just enables it in Xorg::
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setxkbmap us,cz -variant ,ucw -option grp:caps_switch
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Technically, this sets *two* layouts (or *groups*), which are switched by
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pressing the CapsLock key. This has some disadvantages (I have managed a few
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times to have the order accidentally swapped and layout switchers (or their
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users) are sometimes confused), but actual issues are rare. Under X11, that is…
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Corollary 1: I want UCW to work differently.
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My custom keyboard layout?
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--------------------------
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I didn't like fact that UCW layout is in fact just an overlay and not a
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"proper" level 3, so I started studying how to create a custom keyboard layout.
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Few notable resources: `XKB page on ArchWiki`__, manpages for
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`xkeyboard-config(7)`__, `setxkbmap(1)`__ and `xkbcomp(1)`__ and looking into
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``/usr/share/X11/xkb/symbols/cz`` (found by grepping ``/usr/share/X11`` for
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"ucw").
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Another reason for creating my own layouts is various tweaks esp. on laptop
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keyboards. For example, the laptop I am typing this sentence on has broken the
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up arrow key, so I would like to map it somewhere. I use ``xmodmap`` for that,
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but if this could be contained in a single layout, it would make stuff simpler
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for me. (And as I will later learn, ``xmodmap`` `does not work well`__ with xkb
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layouts…)
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Also, I have another tweak on my keyboards: Compose key. This really calls for
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the custom layout! And when at it, I should create one for the ttys as well…
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But this had quite a low priority (the ``kbd.sh`` script in *all* my homes does
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the job well enough), so I didn't get to it in time. Luckily, maybe – if I were
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quicker I would not end up in this rabbit hole. At this point, I knew the basic
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stuff and was reasonably sure that I could hack it together, at least somehow;
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what was stopping me was lack of time and not being sure how I want to manage
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machine-specific tweaks in two different layout syntaxes (xkb for X11 and kbd
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for ttys).
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Aside: A few notes about xkb layouts for completeness
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-----------------------------------------------------
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This all has been described elsewhere in more detail, but it is useful to
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know when debugging layouts, so I will mention it here again.
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There are two forms xkb layouts may be specified. The low-level one is called
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KcCGST (keycodes, compat, geometry, symbols, types) and describes very much
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everything that should happen on any keypress. These actions are described in
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subdirectories of ``/usr/share/X11/xkb/`` of the same name.
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The high-level descriptions are called RMLVO (rules, model, layout, variant,
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option) and are what you usually configure, either with ``setxkbmap`` or using
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GUI tools. Files in ``/usr/share/X11/xkb/rules/`` describe the translation of
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RMLVO to KcCGST (the XML files add human-readable names for the GUIs and the
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like). Maybe confusingly, the RMLVO names of options and layouts are very
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similar to the names of KcCGST compat and symbols, but are generally a
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different thing AFAIK.
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KcCGST also come in two forms: the readable one and the complete one. The
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readable one does not determine everything in one file, instead including
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others. The complete description is, well, complete. It is kind of similar to
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the C preprocessing.
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The readable KcCGST are what is stored in the filesystem as well as what you
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get from ``setxkbmap -print`` and may serve as a good starting point for
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tweaks. To get the complete ones one can use ``xkbcomp $DISPLAY -``.
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The processing of these description is `different`__ in Xorg and in Wayland. If
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I were to bet where the bug is before digging into it, this sounds like a very
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likely culprit.
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A wild friend has appeared
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==========================
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… and he had a problem and asked me if I would have a look into it. Apparently,
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`group switching didn't work in XWayland`__. At first, I wanted to work it around
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by finally creating the Unified UCW keymap™, but I wanted to learn more about
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XKB (from TODO GUIDES), which took me quite long, again. (Studying, working,
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helping with camps *and* fixing layouts takes some time…)
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Also, since the issue is apparently bigger, solving all group switches would be
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a better and more useful solution than just hacking up the Unified UCW keymap
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(even though I want my tweaks to eventually be xkb-based anyway).
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I had very little experience with Wayland until then, mostly because X11 worked
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for me rather well. I tried running Sway few times before, but usually quickly
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reverted to using Xorg for various reasons (Nvidia GPU in laptop, very laggy
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mouse, maybe *this exact* issue with UCW layout) which I didn't feel like
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solving at that moment.
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I wanted to help my friend, though, so I started looking more into using Sway
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on another laptop and reading about internals of input system on `Peter
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Hutterer's blog`__ TODO NAME SPELLING? Occasionally I would randomly google
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(`with DuckDuckGo`__ :-D) for the issue with group switching, just in case…
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The nerd snipe issue
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====================
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`TODO SOMEONE`__ on GitHub found out an interesting workaround: when they
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*disabled* the group switch, the UCW layout *started working*. Complete "huh?"
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moment, I knew I wanted to know more about why that worked. The
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`xkeyboard-common(7) manpage`__ says about more group switching options, so I
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randomly tried switching by Menu key in XWayland (using ``setxkbmap`` even
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though it complained and was supposed to do nothing), and ended up with *both*
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CapsLock and Menu adding diacritics.
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After a bit of mathematic thinking (more in my `comment of the main issue`__) I
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concluded that it was actually working fine, just that the ``SetGroup`` action
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was evaluated *twice*, which would overflow back to the first group when two
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layouts are set (probably the most common case). Mathematic term for this issue
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is "arithmetic in the Z_2 group" :-)
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Corollary 2: A simple workaround is adding the UCW overlay twice, e.g.
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``setxkbmap us,cz,cz -variant ,ucw,ucw`` (and thus changing the algebraic group
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to be Z_3).
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I think that another workaround would be to change the ``compat`` rules for
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XWayland, but it feels nasty to have such a quirk in xkeyboard-config database.
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Changing them for everyone (in ``compat/evdev`` directly) might break other
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systems, so that also does not seem to be a good way.
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Corollary 3: We are fixing XWayland (or the way it processes events from Wayland).
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Learning what happens
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=====================
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I continued reading, this time mostly `who-t's post series about custom
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layouts`__, `core Wayland protocol`__ as well as the source code of various
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tools (``xev``, ``wev``, ``xkbcli interactive-wayland`` &c.) Given that I knew
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very little about the inner workings of the stack, I wanted to find some code
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that would be run and "enhance" it with so many debug prints that I would
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understand what was the state of various parts of the Xwayland's "stacked xkb".
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I kind of knew that one part of this is *somewhere* in the XWayland server,
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whose code felt intimidating, so I wanted to determine where to start nudging
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it from what APIs the clients use.
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.. tip::
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Downloading various packages in Linux is rather simple, as well as their
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rebuilding. In Arch, I can get the package's PKGBUILD just with ``yay -G
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package``, build it with ``makepkg`` and install it using ``yay -U
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package-….pkg.tar``. Makepkg also has some options which allow me to tweak
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the source code after downloading it and before building it.
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Other distros are probably similar, e.g. for Debian-based distros one can
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use ``apt source``, ``debuild`` and ``apt install`` to do the same.
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One interesting observation is that contrary to my expectation, XWayland does
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not seem to use libxkbcommon (according to the ``/proc/PID/maps`` file). This
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can have several reasons, but its source code also contains a slightly tweaked
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version of (Xorg-style) xkb, which might mean I will be dealing with `the ugly
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code`__ :-/ (Actually, XWayland might not process key events itself, instead
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just passing them to clients, but this seems inconsistent with the issue – what
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else would be introducing the second group switch?)
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.. understanding xev, XKeyEvents &c.
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.. Note: neither xev is very barebones and does very little postprocessing of
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events. (naturally.)
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.. X11: state: 0x2000, sway: 0x4000 or 0x0. These state bits are *not*
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mentioned in libX11 docs… (bits 13 and 14)
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.. TO-MENTION:
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- nested compositor issues
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- the IM resolves compose? (I could bisect it, but too lazy)
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- copy my issue comment in case link rots.
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- list of all the packages I had downloaded and compiled
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