MULTIPLAYER - When Host changes In-Game Video Settings on Local Machine - It causes the Screens of all Connected Players to Flash

We do not have enough information to find the cause of this issue.

Please record a video of this happening while the F3 debug screen is open and attach it to this report.
If you are on Windows, you can use Windows+Alt+R to open a built-in app for recording game footage.
If you are on Mac (Mojave or later), you can use Shift+Command+5 to open a built-in app for recording your screen.
In case you don't have a program to record videos, we recommend using the free recording software OBS.
In case the resulting video file is too large to be uploaded to the bug tracker directly, please upload it elsewhere (e.g. as unlisted video on YouTube) and link to it here.

_{This issue is being temporarily resolved as Awaiting Response. Once the requested information has been delivered, the report will be reopened automatically.}

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So I have to somehow coordinate with everyone connecting to my World... to have them all Record Video, simultaneously with me... and then somehow edit all these videos together into a single video with identical timestamps showing the effect happening on their screens... let alone get permissions to use their personal videos...

Keeping in mind this is not something that can be captured with software like OBS, and instead would have to be physically recorded with a CELL PHONE or Video Camera, because it is the SCREEN that is flashing... not their Windows or Computer.... and coordinate this with a video of myself, physically working my Mouse to change my local video settings.

I'll need some cameras and camera tripods as well, and have them shipped to some people with instructions on how to set them up/use them to physically record this happening on their screens... or at the very least maybe a number of small stands their cell phones can be placed upon and set up to physically record their screens.

Are you going assist me with the financing on all this?   Also I do not own the type of video editing software that would be needed to "edit all these videos" together into a single video... nor do I have the expertise on how to do so, and I may have to go take a video editing class/course... will you pay for this too?

Keeping in mind this is not something that can be captured with software like OBS, and instead would have to be physically recorded with a CELL PHONE or Video Camera, because it is the SCREEN that is flashing... not their Windows or Computer....

Are you sure it's a Minecraft issue? It's hard to believe or diagnose the problem without any media and just you saying "trust me, it's flashing for someone else when I make an adjustment in my slider".

If this doesn't show up on a screen recording, it's likely to be some issue outside of Minecraft. In that case please make sure that your hardware (especially graphics card and monitor) is not faulty and that your graphics driver is up to date.

My apologies for the tone of my previous comment.... I was tired from work...

I will be fair and state that no, I do not know for certain that it IS Minecraft specifically... all I can say is that it ONLY happens with Minecraft v1.21... but did not happen with Minecraft v1.20... and more specifically, that it only happens with Minecraft... and not with any other game.

I can also state that, I only recently started using AT&T Fiber with their provided Nokia BGW320 Modem/Router... where as previous, I was using Xfinity with my own store bought broadband Motorola Modem and a TPLink Router... and with Xfinity, I did NOT have the same issue... but that was also before the v1.21 update.

So for all I know, maybe its an issue with AT&T and their Hardware???

Regardless, it only happens with Minecraft v1.21... but did not happen with Minecraft v1.20... and not with any other Game.

I'll see what I can do about trying to get some video of it happening... I'll ask someone if they can capture it with OBS... but it still may take me some time to coordinate... as I'm still not quite sure how I can show it happening while having it synchronized with what I'm doing on my screen.

...oh and also, since Graphics Driver was mentioned... you are also correct in presuming that it could also be an issue with AMD.... you guys HAVE already confirmed on YOUR end that the Minecraft Loading to Black Screen issue with the latest AMD Driver, is not a problem on your end, but is in fact an issue with AMD's Driver...

Or maybe this screen flashing, is an issue between AMD and nVidia since someone connecting to me, and experiencing this, is using an nVidia video card? 

I dont know, but you're right, it could be anything.

We don't need a recording of everyone on the server, it's enough if just one person can record it happening and you attach the video to this report. Can you also check if this happens now with 1.20.6 or a previous version with your new modem/router?