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

Yes, if macOS offers a brightness slider for the external display while HDR is on. Lower it in Control Center. This adjusts the brightness of ordinary desktop content within the HDR presentation. The monitor’s backlight stays unchanged. This dims SDR content instead of amplifying it 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. Some HDR displays do not offer this adjustment.

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…. If you enable it, combined brightness uses supported hardware brightness for this part of the range 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.