qmk/qmk_firmware

[Bug] Random input lag delay when mousekey feature is on

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#25835 opened on Nov 28, 2025

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bughelp wanted

Description

Describe the Bug

If mousekey is on, keyboard latency is completely random. But if define KEYBOARD_SHARED_EP = yes, this fixes issue, so it might be related to the mouse endpoint. Tested on multiple QMK keyboards, the same random delay behaviour is found on all keyboards with mousekey enabled. All measurements are done via keyboard latency tester. Here are my measurements with 2 variables: mousekey and keyboard shared ep:

Keyboard Used

darkproject/kd83a_bfg_edition

Link to product page (if applicable)

No response

Operating System

Any

qmk doctor Output

Is AutoHotKey / Karabiner installed

  • AutoHotKey (Windows)
  • Karabiner (macOS)

Other keyboard-related software installed

No response

Additional Context

No response

Contributor guide

Tech stack
c
Domain
backendperformance
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
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.
mostly clear
Prerequisites
Basic knowledge of QMK firmwareUnderstanding of USB HID and endpoints
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
The issue indicates that enabling KEYBOARD SHARED EP resolves the random latency. Investigate the mousekey feature's handling of USB endpoints, particularly in `tmk core/protocol/usb descriptor.c` and `tmk core/protocol/usb util.c`. Measure the effect of different USB configurations using the keyboard latency tester. Consider whether the mouse endpoint is causing delays due to polling or shared endpoint conflicts.