I noticed this on the Alpha and it is present on the Beta. Seems a Gnome thing...
When I move a window that is Gnome based or CSD from one screen to the other, my secondary monitor goes black for a moment.
Worse, sometimes, it says no signal and just stops working until the offending CSD window is closed. Which is not always easy to do when it is open on the monitor that just cut out.
I have tried various searches of the web, but no luck so far.
These aren't your issue but they're the closest I can find. I'm pretty sure you've tried them all but may as well list them:
Change refresh rate (and others).
Change driver: 1 , 2.
Use Nvidia setting instead of Gnome control center.
Cables.
Some things don't play well with dual GPU integration, such as what we see in notebooks. And it doesn't really matter if its a low end or a top end notebook either. If it has dual GPU's, you might run into issues. I concur with Carmar, switch to Nvidia GPU at all times if you can.
Also, there is a command to force use of the Nvidia GPU, and to stop use of the Xorg switching Daemon for the Intel GPU. I don't know which one would be right for your situation though.
Dual monitor, not dual GPU. I am on integrated Intel graphics only.
Hey, hope you are doing well, despite the issue you are facing right now.
Have you looked up your CPU SPEC sheet to find out what your chip is capable of, especially in the graphics department? You are most certainly having a strange issue, and I am so baffled, as I am not sure if its a hardware issue, or a software related one.
Lets hope the kernal version is playing nice.
I have two VGA monitors but only one HDMI port and one VGA port. So I have an adapter cable connecting the HDMI from PC to VGA on monitor. I just did some experimenting- it only happens when moving Nautilus from one window to the other. The secondary monitor (HDMI attached with VGA adapter) blanks out.
I switched the cables only, and then the primary monitor hooked up with the HDMI to VGA blanked out.
Z15 does not do it; Z16 does.
So narrowing it down... It's driving me nuts.
It was the following entry in my theme .css:
.nautilus-window .sidebar list { background-color: transparent; }
I am not sure why this was having the effect - and I need to learn more about it. But removing that solved the problem. Very strange...
Very strange indeed sir. I say it's so strange that Dr. Strange couldn't solve this level of strange!
Well, I'm glad you were able to figure it out anyways. Could have something the way Zorin handles theming in OS16.
It is... and though Nautilus is now behaving- gtk3-widget factory has started doing it. 
At least I seem to be on the right track...
It did not used to behave this way. I really think something in Gnome has changed.
Gnome...
All themes are now behaving properly and the flickering is gone. Strangest thing I ever saw.
Cannot thank @337Harvey enough for showing me a great way to edit multiple .css files all at once.
I have 24 other themes that will need tedious editing as it is and this new flavor to the mix would have had me throwing in the towel.
I am still not sure of the cause. The problem was Not present on Cinnamon desktop, Mate, XFCE4... Nor was it present on Gnome on Ubuntu 18.04, Ubuntu 20.04 or... 21.04. Present in Zorin 16 alpha and beta (Gnome). And in Zorin 15.3.
Fixing it - I changed all linear gradients to use percentage Stops instead of repeat colors. That's it.
I am at a loss how that would have that effect. I was thinking Carmars suggestion of screen refresh rate may be involved...
Did Z16Alpha also do it?
Yes, and I was wrong, earlier. Zorin 15.3 on a test install does it (I just had not noticed it before since I normally use gtk3-nocsd). Zorin 16 Alpha and Beta both do it.
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Well, I am not a programmer, as its too complex for my puny little brain to comprehend. But what I do understand from simply running terminal code, is just 1-character off, can make it not work, and I am pretty sure coding is exactly the same way.
I do not have the patience for any of it! lol So good work getting it fixed. Truth is, you make such beautiful themes buddy, I'd hate it if you suddenly stopped making them, especially since you like to make them.
Mine as well. I had a recurrence of it. And this time it blinked only once for a moment- on a non csd app.
Earlier today was the first time it did it on a noncsd application. This tells me that although csd does it consistently and that changing my theme does it consistently... It is not limited to themes or csd.
It also tells me that although it is counter-intuitive... csd apps require more resources than non-csd ones.
I started playing with a theme and I entered a lot of values for hex color #555555. With each addition of One Color, I tested the window until adding ONE instance of #555555 caused the screen to blank out (csd app). I removed it from the theme, no blanking. Re-added it - blanking. Removed- no blanking. Re-added- blanking. I did it like twenty times to be sure.
Results:
Recently, I bought Two New (cheap) Acer vga monitors. My PC has one VGA port and one hdmi port. I think that the cable from hdmi to vga is getting a stronger signal. The hdmi port is sending more information than the vga connection can assimilate- my screen blanks. The signal is going through a bottleneck.
My Older set up (a couple months ago) was a notebook computer with built in display and second monitor was one HDMI to HDMI monitor.
This is why I did not experience it on Z15 before, but do now. I do not know why I do not experience it on Ubuntu.
Interestingly, calling on many different rgba or hex colors for a theme seems fine- but calling on the same rgba or hex color seems to put more strain- more information in the signal. This is also counter-intuitive: My older beginner themes that used a Lot of PNG images do fine. But the newer ones with more linear gradients and radial gradients send More Information to the monitor than PNG images do. I would have thought that this was the other way around.
So my newer themes take up much less space on the drive- but require more resources to run.
This explains why the some of the Zorin themes also do it- as small as they look.
There is always a GUI theme. Always. So I cannot test "themeless" without just running in tty which would tell me nothing. But while initially very annoying, there is a silver lining. This has taught me a lot about how it all works and what limitations may exist a person may not expect - allowing me to maximize what I am making. The downside is- I need to redo all of my made themes- which will take a while. Though they will work fine on the vast majority of set ups... It's not enough to just leave it as it is.
And... looks like I need to buy a video card...
Excellent work finding the root cause. Glad you discovered the fix, so you know what's going on. But the last part about you needing a video card, good luck with that, cause those are in short or no supply these days.
I was wrong.
It's the use of hex color #555555. I can use rgba for the same color all I want.
I did some experimenting. I took a known working theme and changed ONE background color to #555555. That was the only change I made. Monitor blinked out.
Changed the background color back to original- everything was fine. I tried this with several different themes, even a couple system themes. Same result, each time.
Again, it may be the adapter from HDMI to VGA but... That One Hex Color...
I tried a bunch of different ones to no effect. Except for #555.
Strange.
Yeah, I am sure VGA is going to have a limited color pallet, as opposed to HDMI, which will have no color limits. Its still a really strange thing to come across though.