Hi, For me, xterm behaves exactly like vte, it prints "ac".
Could you please share your xterm version, configuration, and anything printed to the terminal prior to this test that might be relevant? Unfortunately, it seems like this is the expected behavior. Printing a letter to the last column shouldn't advance the cursor, it should only set an "about to wrap" flag. The cursor, as seen in xterm, remains in the last visible column (although vte decides to hide the cursor instead). Backspace, unfortunately, does not take this "about to wrap" flag into account. Most of the terminals that behave like you wish to, i.e. print "abc", suffer from bug https://bugzilla.gnome.org/show_bug.cgi?id=731155, the test ncurses app there incorrectly prints "50" instead of the expected "50%". See also https://gitlab.gnome.org/GNOME/vte/-/issues/62. e.
