Luis Javier Merino authored Feb 25, 2021 and Kurt Hindenburg committed Apr 17, 2021

Since double-width/double-height lines have room for just half the
characters, take that into account.

Can be tested with:

perl -E '$r=join "", map{$_%10}1..80; say "\e<\e[?40h\e[?3l\e[?7h$r";
         $s="L"."X"x38 ."RL"; say "\e#6$s"'

The above tests that appending characters to double-width lines in
DECAWM (Auto Wrap Mode) wraps at the correct last column. There should
appear a second line (single-width) with a single "L".

perl -E '$r=join "", map{$_%10}1..80; say "\e<\e[?40h\e[?3l\e[?7l$r";
         $s="L"."X"x42 ." TEST FAILED OUTOFBOUNDS R"; say "\e#6$s"'

The above tests that appending characters to double-width lines in
non-DECAWM (Auto Wrap Mode) doesn't write past the last column (visible
on repaints - switch to another window and back). The last column (below
columns 79 and 80 of prev line) should have an "R".