In GNU, Heirloom, and Plan 9 roff, tab positions apply to *input* lines,

In GNU, Heirloom, and Plan 9 roff, tab positions apply to *input* lines,
not to *output* lines.  In particular, if an input line gets broken in
fill mode and a tab occurs in the second output line, it advances to a
position of at least (width of the first output line) + (width of a
space character even though this is never printed) + (width of the part
of the second output line that precedes the tab).

Implement the same logic in mandoc.

Again, do not use tabs in filled text: they have surprising effects,
including this one.