> It copies created leading whitespaces at each line shorter than the longest one.
I am assuming that you mean the trailing (towards the end of line) space.
> It is annoying to me: is it intended to be as it is, is it a bug, or a non-existing-yet feature?
I see that copying of whitespace as a feature, as I have needed that many times. It's really helpful when you are pasting a rectangle block of text in between some other text.
Also, as the name says, it copies a "reactangle" of text. Without the whitespace, the block won't be a rectangle.
Look into delete-trailing-whitespace[0] function, which you can even add to the after-save-hook.
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Kaushal Modi