Why does another app's brightness overlay stop at 100%?

An overlay that reads only macOS brightness feedback may not know about BetterDisplay's brightness upscaling or external-display adjustments. To show those values correctly, the receiving app needs to support BetterDisplay's own OSD notifications.

Enable Settings > Application > Integration > Dispatch OSD notifications, then enable BetterDisplay integration in the other app if it offers that option. BetterDisplay sends adjustment feedback, including the value and its maximum, so a compatible receiver can represent the relevant range.

Turning on dispatch does not make an unsupported overlay understand the data. If the receiving app has no BetterDisplay integration, use BetterDisplay's own OSD or ask the other app's developer about support. A system brightness value stopping at 100% does not show that BetterDisplay's extended brightness adjustment has stopped working.

Dispatching notifications also leaves BetterDisplay's own overlays enabled. If both apps show feedback, turn off Show brightness, volume, mute OSD under Settings > Keyboard > OSD to hide BetterDisplay's basic feedback. If you also want to hide feedback for other adjustments, turn off Show custom OSD icons before hiding the basic OSD.

See third-party app integration for setup and the developer notification format, and OSD styles for BetterDisplay's feedback choices.