Can I dim ordinary desktop content while keeping an external display in HDR mode?

Yes, where macOS provides brightness adjustment for an external display in HDR mode, you can lower the SDR brightness using Control Center. This adjusts the brightness of ordinary desktop content within the HDR presentation. It is different from changing the monitor’s backlight or amplifying SDR into the HDR range.

Keep the display in HDR mode and check whether macOS offers its brightness slider. BetterDisplay also uses this system capability as part of combined brightness on supported displays. Availability depends on the display, connection and system support. An HDR label alone does not guarantee it.

If you want BetterDisplay to use that system adjustment, leave Disregard SDR peak brightness adjustment availability off under the display’s Image Adjustments > Image Adjustment Settings…. Also check Allow hardware brightness control in HDR mode under Device Control Settings…: enabling it makes supported hardware brightness take that role in combined control instead. The macOS adjustment remains available separately in Control Center.

Avoid using software brightness upscaling as a substitute when your goal is to preserve HDR highlights while making desktop content dimmer. See HDR highlight clipping and Device Control Settings for the distinction between these methods.