What I'm asking for

Full Steam Input UI support for non-standard gamepad buttons (misc1–misc5, paddle1–paddle4) — so they appear in the layout editor and can be mapped like any other button, not just through "Configure Device".

Additionally: IMU (gyroscope/accelerometer) support for gamepads using custom Linux drivers that expose sensors via uinput/UHID.

Real example: Flydigi Vader 5 Pro on Linux

The Flydigi Vader 5 Pro has 9 extra buttons (C, Z, LM, RM, M1–M4, O) and a built-in IMU. I set up full Linux support via padctl (open-source HID driver):

• ✅ All buttons registered in Steam (verified via "Configure Device")
• ✅ Rumble working
• ✅ IMU uinput device created with INPUT_PROP_ACCELEROMETER
• ✅ SDL_GameControllerDB entry added ([PR #940](https://github.com/mdqinc/SDL_GameControllerDB/pull/940))
• ❌ Extra buttons not visible in Steam Input layout editor
• ❌ IMU not detected by SDL/Steam (uinput has no shared parent HID node — needs UHID support in padctl, tracked in padctl#81)

Everything on my side is done. The missing piece is Steam UI.