# Studio Display Brightness Key Issue — Troubleshooting Summary

## Setup
- MacBook Pro (M5) connected directly to Apple Studio Display — no hub or dock
- No other displays connected

## Issue
Keyboard brightness keys (F1/F2) do not control the Studio Display's brightness. They only affect the internal MacBook display. Brightness *can* be adjusted manually via the System Settings slider and the Control Center / menu bar icon — only the keyboard shortcut fails to reach the external display.

## Troubleshooting steps taken
1. Confirmed brightness keys work correctly on the internal MacBook display
2. Confirmed brightness can be adjusted via the slider in System Settings → Displays
3. Confirmed brightness can be adjusted via Control Center / menu bar icon
4. Checked System Settings → Keyboard — "Use F1, F2, etc. as standard function keys" ruled out
5. Switched to a different Thunderbolt port on the Mac
6. Closed and reopened the MacBook lid
7. Restarted the Mac
8. Updated the Studio Display's firmware
9. Confirmed Studio Display is set as the primary display (has the menu bar) in Displays → Arrangement
10. Checked Activity Monitor / login items for conflicting apps (Karabiner, BetterTouchTool, Logi Options+, etc.) — none installed
11. Logged into a Guest User account — issue persisted, and brightness keys also failed for the internal display in that account
12. Created a brand new standard user account — brightness keys still failed for the external display
13. Updated macOS fully to Tahoe 26.6 — issue persists after update and restart
14. Unplugged the Studio Display's power cable for 30 seconds, then reconnected — issue persists

## Result
Brightness keys never control the Studio Display, across multiple user accounts (main, guest, and new standard account), and after a full macOS update, display firmware update, and full power cycle of the display. The Displays slider and Control Center brightness control both work correctly — only the physical keyboard shortcut fails to reach the external display.