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<title>UCW keyboard layout on XWayland – LEdoian's Blog</title>
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<h2>Stalk me also at</h2>
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TODO!
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TODO!
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<main>
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<h1>UCW keyboard layout on XWayland</h1>
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<p>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.</p>
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<div class="section" id="background-the-ucw-layout">
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<h2>Background: The UCW layout</h2>
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<p>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.</p>
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<p>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,…</p>
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<p>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 :-)</p>
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<p>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:</p>
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<pre class="literal-block">
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setxkbmap us,cz -variant ,ucw -option grp:caps_switch
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</pre>
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<p>Technically, this sets <em>two</em> layouts (or <em>groups</em>), 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…</p>
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<p>Corollary 1: I want UCW to work differently.</p>
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<div class="section" id="my-custom-keyboard-layout">
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<h3>My custom keyboard layout?</h3>
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<p>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: <a class="reference external" href="TODO">XKB page on ArchWiki</a>, manpages for
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<a class="reference external" href="TODO">xkeyboard-config(7)</a>, <a class="reference external" href="TODO">setxkbmap(1)</a> and <a class="reference external" href="TODO">xkbcomp(1)</a> and looking into
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<tt class="docutils literal">/usr/share/X11/xkb/symbols/cz</tt> (found by grepping <tt class="docutils literal">/usr/share/X11</tt> for
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"ucw").</p>
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<p>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 <tt class="docutils literal">xmodmap</tt> 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, <tt class="docutils literal">xmodmap</tt> <a class="reference external" href="TODO">does not work well</a> with xkb
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layouts…)</p>
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<p>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…</p>
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<p>But this had quite a low priority (the <tt class="docutils literal">kbd.sh</tt> script in <em>all</em> 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).</p>
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</div>
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<div class="section" id="aside-a-few-notes-about-xkb-layouts-for-completeness">
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<h3>Aside: A few notes about xkb layouts for completeness</h3>
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<p>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.</p>
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<p>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 <tt class="docutils literal">/usr/share/X11/xkb/</tt> of the same name.</p>
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<p>The high-level descriptions are called RMLVO (rules, model, layout, variant,
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option) and are what you usually configure, either with <tt class="docutils literal">setxkbmap</tt> or using
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GUI tools. Files in <tt class="docutils literal">/usr/share/X11/xkb/rules/</tt> 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.</p>
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<p>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.</p>
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<p>The readable KcCGST are what is stored in the filesystem as well as what you
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get from <tt class="docutils literal">setxkbmap <span class="pre">-print</span></tt> and may serve as a good starting point for
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tweaks. To get the complete ones one can use <tt class="docutils literal">xkbcomp $DISPLAY -</tt>.</p>
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<p>The processing of these description is <a class="reference external" href="TODO">different</a> 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.</p>
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</div>
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</div>
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<div class="section" id="a-wild-friend-has-appeared">
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<h2>A wild friend has appeared</h2>
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<p>… and he had a problem and asked me if I would have a look into it. Apparently,
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<a class="reference external" href="TODO">group switching didn't work in XWayland</a>. 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 <em>and</em> fixing layouts takes some time…)</p>
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<p>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).</p>
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<p>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 <em>this exact</em> issue with UCW layout) which I didn't feel like
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solving at that moment.</p>
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<p>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 <a class="reference external" href="TODO">Peter
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Hutterer's blog</a> TODO NAME SPELLING? Occasionally I would randomly google
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(<a class="reference external" href="TODO">with DuckDuckGo</a> :-D) for the issue with group switching, just in case…</p>
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</div>
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<div class="section" id="the-nerd-snipe-issue">
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<h2>The nerd snipe issue</h2>
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<p><a class="reference external" href="TODO">TODO SOMEONE</a> on GitHub found out an interesting workaround: when they
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<em>disabled</em> the group switch, the UCW layout <em>started working</em>. Complete "huh?"
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moment, I knew I wanted to know more about why that worked. The
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<a class="reference external" href="TODO">xkeyboard-common(7) manpage</a> says about more group switching options, so I
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randomly tried switching by Menu key in XWayland (using <tt class="docutils literal">setxkbmap</tt> even
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though it complained and was supposed to do nothing), and ended up with <em>both</em>
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CapsLock and Menu adding diacritics.</p>
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<p>After a bit of mathematic thinking (more in my <a class="reference external" href="TODO">comment of the main issue</a>) I
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concluded that it was actually working fine, just that the <tt class="docutils literal">SetGroup</tt> action
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was evaluated <em>twice</em>, 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" :-)</p>
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<p>Corollary 2: A simple workaround is adding the UCW overlay twice, e.g.
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<tt class="docutils literal">setxkbmap us,cz,cz <span class="pre">-variant</span> ,ucw,ucw</tt> (and thus changing the algebraic group
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to be Z_3).</p>
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<p>I think that another workaround would be to change the <tt class="docutils literal">compat</tt> 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 <tt class="docutils literal">compat/evdev</tt> directly) might break other
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systems, so that also does not seem to be a good way.</p>
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<p>Corollary 3: We are fixing XWayland (or the way it processes events from Wayland).</p>
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</div>
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<div class="section" id="learning-what-happens">
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<h2>Learning what happens</h2>
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<p>I continued reading, this time mostly <a class="reference external" href="TODO">who-t's post series about custom
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layouts</a>, <a class="reference external" href="TODO">core Wayland protocol</a> as well as the source code of various
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tools (<tt class="docutils literal">xev</tt>, <tt class="docutils literal">wev</tt>, <tt class="docutils literal">xkbcli <span class="pre">interactive-wayland</span></tt> &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 <em>somewhere</em> 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.</p>
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<div class="admonition tip">
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<p class="first admonition-title">Tip</p>
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<p>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 <tt class="docutils literal">yay <span class="pre">-G</span>
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package</tt>, build it with <tt class="docutils literal">makepkg</tt> and install it using <tt class="docutils literal">yay <span class="pre">-U</span>
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<span class="pre">package-….pkg.tar</span></tt>. Makepkg also has some options which allow me to tweak
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the source code after downloading it and before building it.</p>
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<p class="last">Other distros are probably similar, e.g. for Debian-based distros one can
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use <tt class="docutils literal">apt source</tt>, <tt class="docutils literal">debuild</tt> and <tt class="docutils literal">apt install</tt> to do the same.</p>
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</div>
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<p>One interesting observation is that contrary to my expectation, XWayland does
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not seem to use libxkbcommon (according to the <tt class="docutils literal">/proc/PID/maps</tt> 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 <a class="reference external" href="TODO">the ugly
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code</a> :-/ (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?)</p>
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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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</div>
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</div>
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</main>
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</div> <!-- #main -->
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<footer>
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<hr>
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