pinephone keyboard case - plasma mobile - AltGr does not trigger symbols on number row

Describe your issue

What's the expected behaviour?

The pinephone keyboard case's symbols are currently accessible as an xkb "level 3". A key should be configured to trigger level 3 in plasma mobile by default. See !3310 (merged) and !3175 (merged) for previous work.

What's the current behaviour?

Pressing the AltGr key (default in Phosh and sxmo-sway currently to trigger level 3) and typing on the number row does not output the symbols (\ | etc) printed on the keys.

How to reproduce your issue?

Install Plasma Mobile on the pinephone, insert it into the keyboard case, open any text entry field, hold down AltG key, and press a number key.

What device are you using?

pine64-pinephone

On what postmarketOS version did you encounter the issue?

• edge (master branch)

• v22.06

• v21.12 (supported until 2022-07-12)

• I confirm that the issue still is present after running sudo apk upgrade -a

On what environment did you encounter the issue?

Phone environments

• Phosh
• Plasma Mobile
• Sxmo (Wayland/Sway)

Other

• No environment ("none" in pmbootstrap)
• GNOME
• KDE Plasma
• Kodi
• MATE
• Shelli
• Sway
• Weston
• Xfce4
• fbkeyboard
• i3wm
• Sxmo (Xorg/Dwm)
• console UI
• framebufferphone

How did you get postmarketOS image?

• from https://images.postmarketos.org
• I built it using pmbootstrap
• It was preinstalled on my device

What's the build date of the image? (in yyyy-mm-dd format)

2022-07-29

Additional information

Discussion in #1587 (closed) may be relevant. I've done some troubleshooting: #1587 (comment 1045741085)