qmk/qmk_firmware

[Bug] `RGB_MATRIX_SPLIT` + WS2812 behaves wrong on right side

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#25,406 opened on Jun 24, 2025

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bughelp wanted

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Description

Describe the Bug

As diagnosed a few weeks ago (forgot to open this before) at discord

Culprit seems to be that #24364 removed/broke the right-side check for "if index is not within this half's boundaries, do nothing".

This is, with "split_count": [X, Y]:

  • Using an index [0, X) on left side should affect those LEDs: works
  • Using an index [X, X+Y) on right side should affect those LEDs: works
  • Using an index [X, X+Y) on left side should be ignored: works
  • Using an index [0, X) on right side should be ignored: doesn't work, it affects leds [X, X+X) AKA first X leds on the half's chain

Keyboard Used

No response

Link to product page (if applicable)

No response

Operating System

No response

qmk doctor Output

Is AutoHotKey / Karabiner installed

  • AutoHotKey (Windows)
  • Karabiner (macOS)

Other keyboard-related software installed

No response

Additional Context

No response

Contributor guide

Research direction
Inspect the changes in PR #24364 that removed the right side index boundary check. Locate the missing condition in the WS2812 RGB matrix code and re add it to ignore indices outside the half's range.
Tech stack
c
Domain
embedded iot
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
CGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60