Download: here (macOS Catalina required, app is signed and notarized)
The app doesn't automatically check for updates. I recommend selecting "Watch->Releases only" at the top of this page if you have a GitHub account.
This is a small menu bar app to mitigate the problem of the first-generation LG UltraFine not being able to automatically adjust brightness. It will poll the brightness of your built-in display and synchronize it with your LG monitors. So you can use the ambient light sensor of your MBP or iMac for all your (UltraFine) displays! This also means manually adjusting the brightness of all your displays at once is easier, a single swipe on your Touch Bar or press on your keyboard will do.
The difference between this app and some existing apps is that this app uses a private framework of macOS to control the backlight of the LG UltraFine the same way the secondary slider of your Touch Bar will do. Other apps might virtually darken the display with the backlight staying the same, this will greatly reduce contrast.
Because this app relies on your Mac’s ambient light sensor, unfortunately it won't help with automatic brightness if your MacBook is in clamshell mode or you have connected your UltraFine to for example a Mac Mini.
I only have one 27-inch LG UltraFine display, so I could do only limited testing. Let me know if you have issues.
Requires macOS 10.15. If you're on 10.14, you can download v1 from the releases page.
You'll probably want to add the app to your Login Items.
The app polls the brightness pretty aggressively, which results in a small energy impact of around 0.3-0.5 according to Activity Monitor. However it will automatically stop the polling when no UltraFine displays are connected and because those monitors are also a power source, this effectively means it will never run when on battery power.
If you enter/exit clamshell mode by closing/opening your lid with the monitor attached, it might go completely bright for a second before restoring to the last synchronized brightness.