From d3069760d70af899f6c6b213436dc9272c6af752 Mon Sep 17 00:00:00 2001
From: Jens van Almsick <bin101@users.noreply.github.com>
Date: Fri, 25 Sep 2026 20:05:33 +0200
Subject: [PATCH] [OS Detection] Don't classify MacOS sequences as Windows when
 cnt_02 >= 2 (#26303)

fix(os_detection): don't classify macOS sequences as Windows

macOS 26.x (ChibiOS) can send a late wLength=0x04 packet after the two
0xFF packets that already established OS_MACOS. The Windows check
(cnt_ff >= 2 && cnt_04 >= 1) fired on this combination and overwrites
the correct result.

Real-world Windows sequences always start with 0xFF packets (cnt_02 = 0);
macOS sequences always open with at least two 0x02 packets (cnt_02 >= 2).
Adding && setups_data.cnt_02 < 2 to the Windows condition is sufficient
to separate the two populations without affecting any existing detection.

Adds a regression test for the affected sequence.
---
 quantum/os_detection.c                      | 2 +-
 quantum/os_detection/tests/os_detection.cpp | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/quantum/os_detection.c b/quantum/os_detection.c
index 2a5237dfc31..8c78e4c94eb 100644
--- a/quantum/os_detection.c
+++ b/quantum/os_detection.c
@@ -147,7 +147,7 @@ void process_wlength(const uint16_t w_length) {
     // now try to make a guess
     os_variant_t guessed = OS_UNSURE;
     if (setups_data.count >= 3) {
-        if (setups_data.cnt_ff >= 2 && setups_data.cnt_04 >= 1) {
+        if (setups_data.cnt_ff >= 2 && setups_data.cnt_04 >= 1 && setups_data.cnt_02 < 2) {
             guessed = OS_WINDOWS;
         } else if (setups_data.count == setups_data.cnt_ff) {
             // Linux has 3 packets with 0xFF.
diff --git a/quantum/os_detection/tests/os_detection.cpp b/quantum/os_detection/tests/os_detection.cpp
index 21c4536243e..6a1f0b9db3e 100644
--- a/quantum/os_detection/tests/os_detection.cpp
+++ b/quantum/os_detection/tests/os_detection.cpp
@@ -71,6 +71,7 @@ macOS 12.5: [2, 24, 2, 28, FF]
 macOS 15.1.x: [ 2, 4E, 2, 1C, 2, 1A, FF, FF]
 macOS 15.x (another host): [ 2, 0E, 2, 1E, 2, 42, FF]
 macOS 15.x (periodic weirdness): [ 2, 42, 2, 1C, 2, 1A, FF, 2, 42, 2, 1C, 2, 1A, FF ]
+macOS 26.x: [02, 22, 02, 0E, 02, 42, FF, FF, 4, FF]
 iOS/iPadOS 15.6: [2, 24, 2, 28]
 Linux (including Android, Raspberry Pi and WebOS TV): [FF, FF, FF]
 Linux (another host): [FF, FF, FF, FF, FF, FF]
@@ -163,6 +164,12 @@ TEST_F(OsDetectionTest, TestChibiosMacSequoia3) {
     assert_not_reported();
 }
 
+TEST_F(OsDetectionTest, TestChibiosMacosWithLate04Packet) {
+    EXPECT_EQ(check_sequence({0x02, 0x22, 0x02, 0x0E, 0x02, 0x42, 0xFF, 0xFF, 0x04, 0xFF}), OS_MACOS);
+    os_detection_task();
+    assert_not_reported();
+}
+
 TEST_F(OsDetectionTest, TestLufaMacos) {
     EXPECT_EQ(check_sequence({0x2, 0x10, 0x2, 0xE, 0xFF}), OS_MACOS);
     os_detection_task();