strange behavior of TAB when whitespace-mode activated

[Frédéric Baldit]

Hi guys,

I'm experiencing something I don't understand (I came to this while
reading emacs manual): when in Text-mode, using TAB key doesn't lead to
the same result when whitespace-mode is or not activated. Here is my
minimal example:

1) open emacs on Text mode (file named e.g. test.txt) 

2) type the following text ( «.» characters hereunder are used for
spaces, «$» for newline):

....First.line.of.text.in.emacs$
....Second one

3) delete some or all of the four spaces at beginning of line number 2
and put point at beginning of line 2:

....First.line.of.text.in.emacs$
..Second one

4) then, as it should be, typing on TAB key many times
moves automatically the text on line 2 to the beginning of the words of
line 1. For example after hitting four times on TAB we have: 

....First.line.of.text.in.emacs$
..................Second one

(here on second line there may be some TABs inserted)

5) Now, if I proceed strictly identically from step 3), but with
whitespace-mode activated, I have the following strange result after
hitting TAB four times:

....First.line.of.text.in.emacs$
..........................Second one

(here also, on second line there may be some TABs inserted)

I found this both on emacs version 26.1 (old version installed from
debian 10 stable, which is the OS I currently use) and on 27.1.91
(compiled from git sources).

It seems to me it could be a bug, but I'm probably missing something
here, as I'm not an «emacs expert». Any help to explain this thing to
me would be greatly appreciated! (or confirmation that there may be a
bug)

--
  Frédéric Baldit