See my svn commit 6129 for a partial solution: pressing "space tab" inserts a tab (expanded with spaces) of default width 8, which you can configure with

  (set-preference "editor:verbatim:tabstop" 4)

in a scheme session. This works in verbatim, code, etc. Also in prog modes. I chose "space tab" because "M-tab" is used for auto indentation in scheme. It is trivial to change this for verbatim inside text-kbd.scm.

I haven't done anything about the literal tab character. This needs tweaking the converters, defining a new texmacs "glyph" or whatever it's called and then expanding it in the typesetter. I can do the latter if someone takes care of setting up the converters to send some character like "<tab>" to TeXmacs whenever a tab is read from a file. The input from keyboard is then a simple matter of defining a keymap ("space var" (insert "<tab>")) in the desired modes.

I've made the summary more accurate.

I've had many attempts to introduce the standard TAB behaviour, adding a variable width space to the next multiple of 8 characters in fixed width mode, but what I find hard is that the left-box position is always zero.

The bug is probably not n htab which is doing what it is designed to do.

Possible M-TAB should not be inserting htab into verbatim.

However hspace isn't the right thing either, we want to do traditional tabbing to an offset equal to the next multiple of tab-interval.

I suspect texmacs can't do this.

In a verbatim block (and therefore also a session), M-TAB to insert a tab inserts much more than 8 spaces and in fact right aligns the line.