I usually just start an onscreen keyboard but I look forward to smarter folks telling me the way...
I think Wayland Qt apps start the keyboard automatically when I focus a text box. It works in Firefox, for example. Doesn't seem to be the case for Xwayland apps and I haven't tested Gtk.
On my Thinkpad 2in1, the behavior your desire is what I get. I'm on Fedora KDE (Kinoite) which pre-installs Maliit by default though, I'm not sure if it comes with some additional configuration.
The help page of virtual keyboards mentions detecting a hardware keyboard, but that's all I know. Does it not detect the keyboard built into my laptop? It makes no sense to me lol.
How do you start that? Is it a seperate package? I never even saw it but really curious
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
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If it hasn't, report it yourself.
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