Use the gmux backlight instead of the Touch Bar on T2 Macs (#6597)

* Match display backlight candidates against real globs

[[ ]] does not do pathname expansion, so amdgpu_bl* and acpi_video* only
ever tested for files with a literal asterisk in the name. Every machine
without intel_backlight silently fell through to the alphabetical first
entry, which picks acpi_video0 over amdgpu_bl0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Use the gmux backlight instead of the Touch Bar on T2 Macs

/sys/class/backlight on a T2 Mac holds appletb_backlight and
gmux_backlight. Neither was a candidate, so the alphabetical fallback
picked the Touch Bar and brightness keys dimmed it instead of the
display. Add gmux_backlight and never fall back to the Touch Bar, which
is not a display panel on any Mac.

gmux ranks above the GPU backlights because apple-gmux only registers
its device when the kernel has already selected it for the machine, and
on dual-GPU Macs the GPU's own PWM stops driving the panel as soon as
that GPU suspends.

Fixes #6558

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

bin/omarchy-hw-display

@@ -3,11 +3,17 @@
 # omarchy:summary=Print the most likely display backlight device.
 # omarchy:examples=omarchy-hw-display
 
+backlight_path="${OMARCHY_BACKLIGHT_PATH:-/sys/class/backlight}"
+
 # Start with the first possible output, then refine to the most likely given an order heuristic.
-device="$(ls -1 /sys/class/backlight 2>/dev/null | head -n1)"
-for candidate in amdgpu_bl* intel_backlight acpi_video*; do
-  if [[ -e /sys/class/backlight/$candidate ]]; then
-    device="$candidate"
+# Glob the candidates in the loop list: [[ ]] does not do pathname expansion.
+# The Touch Bar on T2 Macs registers a backlight that never drives the display panel.
+device="$(ls -1 "$backlight_path" 2>/dev/null | grep -vx appletb_backlight | head -n1)"
+# gmux comes first: apple-gmux only registers when the kernel has already picked it, and on
+# dual-GPU Macs the GPU's own PWM stops driving the panel once that GPU suspends.
+for candidate in "$backlight_path"/gmux_backlight "$backlight_path"/amdgpu_bl* "$backlight_path"/intel_backlight "$backlight_path"/acpi_video*; do
+  if [[ -e $candidate ]]; then
+    device="${candidate##*/}"
     break
   fi
 done

test/shell.d/hw-display-test.sh

@@ -0,0 +1,64 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
+
+tmp_dir=$(mktemp -d)
+trap 'rm -rf "$tmp_dir"' EXIT
+
+write_backlights() {
+  rm -rf "$tmp_dir/backlight"
+  mkdir -p "$tmp_dir/backlight"
+
+  local device
+  for device in "$@"; do
+    mkdir -p "$tmp_dir/backlight/$device"
+  done
+}
+
+hw_display() {
+  OMARCHY_BACKLIGHT_PATH="$tmp_dir/backlight" "$ROOT/bin/omarchy-hw-display"
+}
+
+write_backlights intel_backlight
+device=$(hw_display)
+[[ $device == "intel_backlight" ]] || fail "the only backlight is used" "actual: $device"
+pass "the only backlight is used"
+
+write_backlights acpi_video0 amdgpu_bl1
+device=$(hw_display)
+[[ $device == "amdgpu_bl1" ]] || fail "globbed candidates beat the alphabetical fallback" "actual: $device"
+pass "globbed candidates beat the alphabetical fallback"
+
+write_backlights acpi_video0 intel_backlight
+device=$(hw_display)
+[[ $device == "intel_backlight" ]] || fail "the candidate order is honored" "actual: $device"
+pass "the candidate order is honored"
+
+write_backlights nvidia_wmi_ec_backlight
+device=$(hw_display)
+[[ $device == "nvidia_wmi_ec_backlight" ]] || fail "an unknown backlight falls back to the first device" "actual: $device"
+pass "an unknown backlight falls back to the first device"
+
+write_backlights appletb_backlight gmux_backlight
+device=$(hw_display)
+[[ $device == "gmux_backlight" ]] || fail "a T2 Mac uses gmux instead of the Touch Bar" "actual: $device"
+pass "a T2 Mac uses gmux instead of the Touch Bar"
+
+write_backlights appletb_backlight
+if hw_display >/dev/null 2>&1; then
+  fail "the Touch Bar is never used as the display backlight"
+fi
+pass "the Touch Bar is never used as the display backlight"
+
+write_backlights acpi_video0 amdgpu_bl0 appletb_backlight gmux_backlight intel_backlight
+device=$(hw_display)
+[[ $device == "gmux_backlight" ]] || fail "gmux outranks the GPU backlights on dual-GPU Macs" "actual: $device"
+pass "gmux outranks the GPU backlights on dual-GPU Macs"
+
+write_backlights
+if hw_display >/dev/null 2>&1; then
+  fail "no backlight device reports failure"
+fi
+pass "no backlight device reports failure"