No worries, I know what it is. Thanks for the video, that made it clear.

The see-through dock background can't be 100% transparent because otherwise mouse clicks would fall through the 'holes' in the icons. It's supposed to be 99.9% transparent, in such a way that it's not visible to the naked eye but Windows still passes mouse clicks to the dock instead of whatever is behind it.

I must have changed something that has the side-effect of this 'transparent' part of the background actually becoming visible to the eye, in a ghostly way.

Since I know what it is, fixing it should be trivial. As soon as I get back to the office, that is.

Ok, now that I had a chance to look at the code, I can see that nothing has actually changed (I thought I might have changed a value when testing for something and then forgot to revert it), so I'm not sure why you are seeing that.

As I explained above, the area immediately behind the icons cannot be fully transparent. If it is, clicking on any part of an icon that is also fully transparent will make Windows send the mouse click to whatever window is behind the dock instead of the dock itself. Furthermore, mouse move events must also be trapped/sent to the dock when you mouse over it, otherwise the dock would de-magnify as soon as the mouse pointer reached a blank space between the icons.

Each pixel in the dock has an RGB color value PLUS an Alpha (transparency) value between 0 and 255. 0 means fully transparent (mouse clicks, etc, go 'through' it to whatever is behind) and 255 is fully opaque.

That 'mask' you are seeing there has an Alpha value of 1, the minimum you can actually give it to still catch mouse clicks. The actual color/RGB value when I first made Nexus was RGB(0,0,0), which is black. However, this did result in what you are seeing in the video, so I changed it a long time ago to RGB(255,255,255), which is white. This made the 'mask', as you call it, invisible to the eye.

Thing is, I can't see it here when magnifying the dock, no matter what background/wallpaper I use, so I don't know why you are seeing it there.

Now, you also state 'I have been using Nexus for quite a long time and never noticed any 'strange anomalies' like this one.' This kind of implies that it wasn't happening before. So what did you change?

Sincerely, I think it would be a waste of time.

But (and I also think this will be a waste of time but at least not so bad) we never know if some obscure combination of settings is causing this, so you could try resetting your settings to factory defaults and see if that changes anything.

Preferences -> Advanced -> Troubleshooting Options -> Reset Everything.

Please don't forget to backup your settings first, so you can restore them later.