extra newlines when pasting altered string in visual block mode

Question

Suppose I have an open buffer containing the following text

1111111111
1        1
1 222222 1
1 2    2 1
1 2 33 2 1
1 2 33 2 1
1 2 33 2 1
1 2    2 1
1 222222 1
1        1
1111111111

I can visually select and yank the 2-rectangle with the command 3gg2l<Ctrl-V>8jey

Then, I can immediately reselect and paste the yanked text (not changing the buffer) with gvp.

However, it seems like as soon as I write to the @@ or @" register (even if I don't change the value), something changes, and pasting doesn't work right anymore.

For example, doing

:let @@=@@
:normal! gvp

changes the buffer to

1111111111
1        1
1 222222
2    2
2 33 2
2 33 2
2 33 2
2    2
222222 1
1  1
1  1
1  1
1  1
1  1
1  1
1        1
1111111111

inserting the contents of the register as new lines, rather than overlaying the visual selection.

What am I doing wrong?

Answer 1

The problem comes from the register type, you can use echo getregtype('"') before and after your let @@=@@ and you'll see that the type has changed.

:h getregtype() tells us that a register can have different types:

"v" for characterwise text

"V" for linewise text

"{width}" for blockwise-visual text

"" for an empty or unknown register

So when you select your rectangle with ctrl+v and then you yank it, you actually set the type of the " register to visual block (Here ^V6 actually)

When you do let @@=@@ you do no respect the register type. Thus you do not past in block wise mode anymore.

The correct way to set a register is to use the function setreg() which accepts as parameter the register type to set. See :h setreg()