Hi,

I would like to design the top part of our UI, but if I wanna move any layer there appears this extremely annoying rectangular outline around it obstructing the view. I can't see, where precisely I'm moving the part .

How to turn OFF this super annoying "feature"??

I don't know how to turn off that box, it's the 'extent of content' indicator which is actually less than one image pixel wide if you zoom in on it, so zooming in may help to stop it from getting in your way.
A trick you can do is to imagine a larger box and place four small blobs of paint at the corners of that box, then when you do the move you'll have a larger content box that does not obstruct your view. After that of course, you'll have to erase the four small blobs that you'd added.

Krita is probably not the best option for precise designs, I mean, you can, but you will likely spend more time figure things out than actually doing a design, Krita is aimed at graphics done more on a free style such as illustrations, paintings, etc. Even Krita doesn't use Krita made icons/UI elements. For this you likely want to use Inkscape (also FOSS as Krita), you can also combine Krita and Inkscape coping vectors to and from.

For what is worth, the moving tool has some controls to position based on coordinates and base on the edges of the objects you are moving, again, good for a quick reposition, not so much for a whole UI mock-up/development.

I thought we originally didn't have that border...but someone seemed unhappy about that so they made a patch to add the border.

it sounds like people want them both ways. We would have to code something in the tool options to be able to turn it on or off. Right now there is no way to toggle that on or off.

While creating tileable textures its intuitive and key feature to have only the graphics visible without any "aids".