SOLVED

QMK bug. :(

NO_ACTION_TAPPING disables layers. Ugh!

See https://github.com/qmk/qmk_firmware/issues/5735

This may be a stupid question, but I'm having trouble getting what seems like it should be a simple MO(1) key to work. I've searched for similar problems, but every post I've found was for more advanced usage of layers; I'm using an absolute basic "hold the FN key" layer application, and it just refuses to work.

I based my split keyboard loosely on handwired/split89, though I've made several modifications. Link: https://github.com/jurassic73/split89

All the keys (except for the FN key) on both halves of the keyboard work; the serial link works; I only have one physical FN key that's on the right half of the keyboard, though I've tried shorting the other MO(1) keys in the keymap (see link below) manually to see if any of them work. No luck.

I've tried various combinations of flags in config.h, and nothing seems to help.

I don't need a full solution, per se, as much as ideas as to where to look for one. I'm a software engineer wit 30 years experience, including C/C , IoT, and driver-level coding, so even a slight hint as to where to look should be enough to push me in the right direction.

The next thing on my list to try is to remove the unused MO(1) keys in case having three of them in the layout (even though two aren't physically wired up) is somehow breaking things. But that seems somewhat unlikely to be the problem, and so I'm reaching out in case there are other directions I should be looking. Specific keys in my second layer that are causing it to fail, perhaps? I guess I'll try that next, but if anyone knows why one or another key would cause the layer to fail, I'd appreciate a hint.

Relevant source in a pastebin here: https://pastebin.mozilla.org/WqophUTz

Ugly picture so you can have a rough idea of what I'm going for: https://photos.google.com/share/AF1QipN_v7uh0gWM_mgZ1-25TdYQ3LEKP3j68TTrlEdWrE3A_KrMPfU8pyjpzUYSl_filg?key=bmZwNS13dnR4ZUFMZDBOYkExRHVVRlhtaFNoaXhB

After everything is working I'll make a post talking about the build etc., and I'll take and add prettier pictures, including the ugly soldering and wiring job on the bottoms. Thanks in advance for any hints that nudge me in the right direction.

Update: I've tested the FN key with a normal key. Hardware works. I've tried removing all but one MO(1) key. Still doesn't help. I've removed all special keys from the second layer. Still doesn't help. Next trying the debug view.