I believe I found the solution. I disabled the GPU.

Under Photoshop->Preferences->Performance, I unchecked the box next to "Use Graphics Processor". Restarted PS and it is working correctly. Smart filters apply the changes as expected, :), same with he filter layers, and no more weirdness with colors changing when changing window size.

I recalled there was a problem with LR and disabling the GPU was the solution. I so I tried it. Maybe it's a problem with ACR? I'm just happy to have PS back. :) And now I have a good excuse to buy that new 16" MacBookPro.
(AS IF I could afford it).

Thank you all for you time and help.

You will never be able to directly open 6DII RAW files (even though they are CR2) in PS CS6. This is because you need Adobe Camera Raw (ACR) 9.12 or later to handle the 6DII's files.

Unfortunately, the very last version of ACR that works in PS CS6 is 9.1.1. So there's simply no way to update CS6 sufficiently to work directly with the 6DII's RAW files.

HOWEVER.... PSE 2021 definitely should be able to work with those 6DII files. It's the latest version of that software, released less than 6 months ago, and has a much later version of ACR built in. I checked and Adobe shows it fully compatible with 6DII. If you can't get it to work, maybe you should ask Adobe to give you your money back!

If you do get Elements 2021 to work with the files, there may be another solution. However, one of the reasons to shoot RAW in the first place is to be able to import and work on 16-bit files. Elements is limited in how much work can be done on 16-bit files. (In fact, Elements wants to default to opening 8-bit files in the RAW converter... you have to check 16-bit to have it make those instead.) So, beyond doing the initial RAW conversion with it, you may not want to do further image editing with Elements 2021 (since you'll have to "downgrade" to 8-bit images to make full use on it).

What you might try instead - if you can get Elements 2021 to recognize the 6DII files - is use that to simply make and save a 16-bit TIFF... which you then transfer to or open to work on in PS CS6. In other words, all you are doing with Elements 2021 is the basic RAW conversion into a 16-bit TIFF.... Then the actual editing and finishing of the image would then be done in CS6, which has quite a bit more capabilities than Elements in dealing with 16-bit files. If preferred, it might also be possible to save a 16-bit PSD file from Elements. I really don't know. Try it and see. That also should be workable in CS6 (though it may be necessary to set some sort of backward compatibility).

As already mentioned in previous responses, another solution if you want to continue to work with PS CS6 is to download and use the free Adobe DNG Converter. That will convert your CR2 files into DNGs (Adobe's "universal raw" file type), which you then can work in CS6. (Note: I recommend you keep the original CR2 files too. There may be a time in the future when you need them, such as working with programs other than Adobe's.)

Finally, depending upon your computer's operating system there may be some limitations on CS6, Elements 2021 and even on Adobe Raw Converter. For example, I know 64-bit Windows 7 and later is needed for full functionality of CS6. There are probably some similar limitations with Mac OS versions, though I'm not as familiar with those.

EDIT: Do you have Lightroom 6? The later versions of that also support 6DII. The final version 6.14 includes ACR 10.1, which is able to handle a lot more Canon DSLRs (and M series mirrorless) than the final version of PS CS6. For example, among the full frame DSLRs all but the very latest 1DXIII are fully supported by LR6 (6.14). Neat thing about LR6 is that it does exactly the same thing as ACR in PS (it just looks different... many think it's easier)... Plus there's a1-click method of "sending" images to PS CS6 for further work (simply right click on the image in LR6 and a little menu pops up that includes an "Edit in" option, one of which can be PS CS6).

A substantial part of our disagreement with each other on this thread is that we have not stated what we are holding constant.

For example, when talking about comparing a full-size sensor and a cropped sensor, the adage that the more light received by the sensor, the less noise there will be. This is true ONLY if one uses the same sensor. In fact, from this scenario, one can infer the correct rule: Each pixel on a given sensor will under normal conditions keep the same level of noise and thus the more light received by a pixel the more the signal-to-noise ratio will increase.

Thus since we really need to look at the characteristics of each pixel on the sensor to evaluate the sensor's dynamic range, we see that (unless we are moving to the extremes in light levels) noice does not have a major effect on dynamic range. I am not sure how or what William J. Claff is actually measuring on his web site, "Photons to Photons," when he is talking about dynamic range, but the conclusions seem incorrect to me.

Also, comparing a full-size sensor output printed on an 8 X 10 paper to a half-size sensor print-out printed also to an 8 X 10 paper, is not good for comparing dynamic range or other pixel-centered characteristics.

Just some thoughts. --Richard