Thanks for replying! I keep forgetting about this, until I try to type anything. And I am brutally reminded.

The terminal I use most often is Yakuake, based on Konsole. The issue also appeared on Konsole. Let me try xterm.

It seems like something I should have thought of myself in retrospect, but xterm works completely as expected. I'm  guessing that xterm uses neither Qt nor Gtk, right? Could the problem be with Qt? If so, why would it also be present in Firefox. And how would I diagnose that?

The problem does not seem to occur in the Linux console, the one you get without a desktop environment launched or by using the ctrl+alt+F-key combo. (By the way, the screen on which Wayland shows up seems to vary. Currently, it's screen 2. Sometimes it's screen 6. Screen 1 only shows a blinking cursor. The Arch docs said Wayland should show up on screen 1. This is not a problem, I'm just mentioning it in case it's a useful detail.) More testing has reminded me that, in the Linux terminal, another keyboard layout is in use: dvorak-fr. That layout is similar but not identical to french variant dvorak. Among other differences, dvorak-fr does not use dead keys.

That why I  noticed that dead keys don't work well in Wayland, neither in Yakuake or in the Firefox, which I am currently typing in. My keyboard layout makes heavy use of them. Switching to the en_US keyboard layout does not change the issue with the arrow keys or with the return key. The dead keys work as expected in xterm. That really puzzles me. Not just that, but xterm is also using the correct layout: french variant dvorak.

What I get with cat (down, enter, left, enter, up, enter, right, enter) changes often. Currently, it's: d s w e. On other recent attempts, the printount has included w, q, nothing (or maybe a dead key), W (that's capital w), and, I think, "home".

If I change the layout to the US keyboard, I get (going round a few times): y, t, space, dot, u, y, space, dot, u, y, space. (I just typed return out of reflex, and it printed m instead.)

Please let me know if there is anything else I can do to figure out what the problem is. My guess is that there may be an issue with the character set or the keyboard model, even though as far as I know, they are both set correctly: utf-8 and standard 105-key keyboard. But maybe they're not set correctly everywhere.