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Just found this design and I think it's totally awesome. The new super-small switch PG1316M is used to fit 3 rows into such a constrained space.
Wonder if you could even get away with 4 rows of this, especially to somewhat compensate for the lack of thumb keys? I'd personally be interested to try 4 rows even for a more conventional tabletop keyboard.
EDIT: I have accidentally erased the main URL when I tried to upload a picture. Apologies. (Didn't know Lemmy works this way). Here's the main URL for this thing: https://www.reddit.com/r/ErgoMobileComputers/comments/1npnj85/btyp_a_mini_pg1316m_switch_keyboard_for_use_on_a/
It doesn't look too bad on the video actually, check it out. (See the post's EDIT, I messed it up at first.)
Depends a lot on your phone size, hand size etc though, I guess. Generally very small keyboards CAN be ergonomic, as far as I've heard (also from people with literally diagnosed RSI). How this one objectively fares IDK
I think my hands are fairly normal sized for a guy, and when I turn my 6.1" iPhone11 horizontal and try to simulate this for 10s I can feel my wrists and forearms tighten and get sore. I don't think I could type on this keyboard for more than 20-30s without RSI pain starting.