2018 Mac Mini bottom half of the display image shacking

OK that is good info.

FWIW the 2018 can support both displays from the USB-C ports.

see > https://support.apple.com/guide/mac-mini/connect-a-display-apd8e4fbbb97/mac

Again to help sort out this problem, I would test a different combination of connections.

for example a USB-C to DisplayPort cable or two. see > usb-c to displayport cable

Plus not only is the USB-C to DisplayPort connection more stable than the HDMI connection, but a it is far cheaper than testing another HDMI adapter and cable. see > https://www.amazon.com/uni-DisplayPort-Thunderbolt-Compatible-More-Gray/dp/B07CL31FR6/ref=pd_lpo_2?pd_rd_i=B07CL31FR6&th=1

Sorry next question. How is the Dell 2408WFP connected to the 2018 Mac Mini?

If HDMI, then I suggest

1. testing a short premium certified HDMI cable. (Mac's can be picky about HDMI cables)
2. or better yet, moving that Dell to a USB-C port by using a USB-C to DisplayPort cable.

sonichanxiao wrote:

2018 Mac Mini bottom half of the display image shacking

Do you mean shaking, flickering or something else?

This mainly happens to websites like Youtube in Chrome or Excel document. Not always, but after using the machine a while, it will happen. Maximize youtube to full screen solve the issue. Not sure what is wrong? Apple store couldn't find anything.

What displays?

The backlighting in that display could be failing.

FWIW I have a Samsung that flickers when it re-wakes are a brief sleep, but then is fine after that.

And not sure if it is related. Sometimes when wake up the machine from sleep, in dual display setup, one of the display would not have any signal. The only solution to fix this is to shut down(restart won't work).

Setup a Hot Corner to "Put Display to Sleep" then when one display fails to light, put the displays back to sleep using the Hot Corner and re-wake them. It will save on Unplugging/Plugging and Shutting Down and Restarting.