Please create a brightness slider item for the panel which can be added to the panel and when clicked on will show a slider to adjust the screen brightness.
I tried putting my mouse over the power manager panel item and the pulse audio panel item and nothing happened, I tried clicking on the power manager panel item and there was no slider option available. I tried this on Xubuntu 20.04 and Manjaro 20.2
When you scroll over the PulseAudio Plugin the volume should change. By scrolling over the Power Manager Plugin the brightness of the screen changes on my system, which is a notebook. If you have a desktop PC I think it is not possible to change the brightness with the plugin.
I do have a desktop PC. Where could I go to make a feature request to support a desktop monitor? Xfce is great but having the brightness at full blast is hard on the eyes.
I think xfce4-power-manager should be the place, but I'm not sure if this is possible for an external monitor. If it is connected over HDMI to your PC, I think there is only video and audio data transmitted, but no control data, like brightness control. In this case brightness changes should, from my knowledge, only be possible over the buttons on the monitor.
You can also try to change the brightness with a keyboard shortcut with xbacklight, maybe this can help you.
I can control secondary display with brightness-controller. Unfortunately that also installed 500mb of dependencies (mostly QT and pyqt bindings). Anycase, technically its possible to adjust external display brightness...
On a laptop, I have connected another screen via HDMI. The internal screen brightness I can switch via Power Manager Plugin, but setting the external screen brightness isn't currently supported. Please add that.
...which is not the physical hardware brightness but only setting software brightness by changing the gamma values.
Hardware is controlled for the internal display via:
Additionally you could also display all controlled displays in the devices list. Currently the devices list is only about battery levels, but it could also list other controlled devices such as displays with used interfaces and max/min/current brightness levels.