Intel HD Graphics eyestrain and using laptop without Power Supply as solution

Could it be noise from the AC power grid? Look up ground loop. It mainly affects audio but maybe it could make your led screen flicker.
I use to DJ for a company and they would pull off the ground prong off the laptop ac power plug. This would solve erratic clicking noises in the speakers/amps. For safety it might not be the best idea to pull off the ground prong. I do know the audio side of things you can get A/V adapters (ground loop isolator) that would flip the phase. This solved the issue. Perhaps there's a similar thing for AC power plugs? https://www.amazon.com/Ebtech-Hum-Plug-Style-Eliminator-Cleaning/dp/B07W682STV
??
Obviously I'm no electronics engineer.

magiks When I take closer look to my display (now i am using old Lenovo T430, but I've observed same with other newer laptops aswell) i see clear difference in colour "production" between power supply on and off

I have the same issue with my (otherwise 100% usable) 2012 Lenovo IdeaPad Yoga 13, but on my laptop the issue surprisingly also extends to the USB ports. In both cases, if power is plugged in or if any USB accessory is plugged in (even if on battery), the same effect will happen, the screen will feel "noisy" and unstable closer to the "same side" the port in use is on and I'll get eye strain on that side of my face

If no ports are in use, the screen feels completely still

I've recreated this many times at this point. The improvement I get in the specific case where I'm running on battery AND zero USB devices are connected is very immediate and noticeable

And no, it's not "emf" related -- because the most comfortable way to use it is actually with the built-in WiFi turned ON and a Bluetooth mouse connected. Wireless connections only is best

If I use a wired Ethernet adapter or a wired mouse instead, that's when the eye strain actually starts happening, because a port is in use

(This laptop only supports Ethernet over USB. There is no physical Ethernet port)

This only happens for me on this laptop. Given this thread, it also might extend to other Lenovo laptops from this era. If I use e.g. my Surface Pro 4 instead, the screen on that tablet feels no different regardless if there is a USB accessory plugged into it or not

My best guess to what is happening here (especially because I've taken apart the computer a few times and noticed that the internals look pretty messy and poorly isolated) is that some power needs to be redirected from the screen to power any active ports, which causes the screen to either flicker or introduce some other kind of display artifacts on the same exact side

I don't think I'm "feeling anything directly from the ports themselves", but more that the ports are affecting the screen in some way

I've also ruled out "getting visually distracted beacuse something is plugged in and sticking out of one side" because if I have a USB accessory right next to that side but leave it unplugged, I do not have this issue! It's only when it's actually plugged in

I also have my Bluetooth mouse and water bottle next to my laptop all the time and I don't get distracted by those

All I know is that if I stay wireless/bluetooth only is when it actually becomes noticeably MORE comfortable!

photon78s In the middle of my post I have already stated that my 2012 Yoga does not have any physical Ethernet port. It only supports Ethernet connections over USB

I also tried two different Ethernet adapters, it doesn't matter. It's simply that whenever either any USB port is in use with any kind of accessory (or the power port is connected), the screen gets worse and more flickery feeling on that side

This doesn't seem to happen on any of the other laptops I own

The only ports on this laptop are USB/HDMI/SD/power/headphones

photon78s probably not it seems to be related to interference with the connection to the internal display itself

because the display starts feeling flickery on the same side the USB or power connection is on. there are USB ports on both the left and right side, and when I switch which side is connected I can actually feel the eye strain shift sides as well.

when nothing is connected at all, as long as dithering is also disabled, the strain totally stops (aside from the mild max brightness PWM, which is subtle enough to not affect me aside from very tiny eye pressure)

I also stop feeling the strain if I turn the screen off/look away for a bit, even if e.g. power is still connected. this one is why I'm pretty confident in my theory that it's related to poor electrical isolation that is "slightly leaking over" into the internal display connection and causing pixel jitter

as mentioned before, WiFi and Bluetooth don't affect this at all. I can connect both a Bluetooth mouse and AirPods, listening to music on them, and streaming remote desktop at high bandwidth over WiFi and feel totally great looking at the internal display for hours, as long as no ports are in use!

(i assume the components for those are further away from the display, or simply have better isolation than the ports do)

imo this laptop probably has no issues with external monitors (as long as the internal display is off). I don't think this issue will affect that. my old monitors all have FRC though so I can't reliably test, but the picture looked normal and pretty clean aside from the annoying FRC patterns each of my monitors always have