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+# Add `autoload promptnl' to your .zshrc, and include a call to promptnl
+# near the end of your precmd function.
+#
+# When promptnl runs, it asks the terminal to send back the current
+# position of the cursor.  If the cursor is in column 1, it does nothing;
+# otherwise it prints a newline.  Thus you get a newline exactly when one
+# is needed.
+#
+# Of course this can make it appear that `print -n' and friends have
+# failed to suppress the final newline; so promptnl outputs the value
+# of the EOLMARK parameter before the newline, with prompt sequences
+# expanded.  So you can for example use EOLMARK='%B!%b' to put a bold
+# exclamation point at the end of the actual output.
+
+# There's another way to accomplish the equivalent, without reading the
+# cursor position from the terminal.  Skip to the end of the file to see
+# that other way.
+
+emulate -L zsh
+
+# VT100 and ANSI terminals will report the cursor position when sent
+# the sequence ESC [ 6 n -- it comes back as ESC [ column ; line R
+# with of course no trailing newline.  Column and line are 1-based.
+
+local RECV='' SEND='\e[6n' REPLY=X
+
+# If you are on a very slow tty, you may need to increase WAIT here.
+integer WAIT=1
+
+# Make sure there's no typeahead, or it'll confuse things.  Remove
+# this block entirely to use this function in 3.0.x at your own risk.
+while read -t -k 1
+do
+    RECV=$RECV$REPLY
+done
+if [[ -n $RECV ]]
+then
+    print -z -r -- $RECV
+    RECV=''
+    REPLY=X
+fi
+
+# This is annoying, but zsh immediately resets it properly, so ...
+stty -echo
+
+# Output the SEND sequence and read back into RECV.  In case this is
+# not a terminal that understands SEND, do a non-blocking read and
+# retry for at most WAIT seconds before giving up.  Requires 3.1.9.
+# For 3.0.x, remove "-t" but don't call this on the wrong terminal!
+
+print -n $SEND
+
+integer N=$SECONDS
+while [[ $REPLY != R ]] && ((SECONDS - N <= WAIT))
+do
+    if read -t -k 1
+    then
+	((N=SECONDS))
+	RECV=$RECV$REPLY
+    fi
+done
+
+# If the cursor is not in the first column, emit EOLMARK and newline.
+
+(( ${${RECV#*\;}%R} > 1 )) && print -P -- $EOLMARK
+
+return 0
+
+# OK, now here's the other way.  Works on any auto-margin terminal, which
+# includes most terminals that respond to ESC [ 6 n as far as I know.  It
+# prints a line of spaces exactly as wide as the terminal, then prints a
+# carriage return.  If there are any characters already on the line, this
+# will cause the line to wrap, otherwise it won't.
+
+: setopt nopromptcr
+: PS1="%{${(pl:COLUMNS+1:: ::\r:)}%}$PS1"
+
+# On a very slow connection, you might be able to see the spaces getting
+# drawn and then overwritten, so reading the cursor position might work
+# better in that circumstance because it transmits fewer characters.  It
+# also doesn't work if you resize the terminal.
+
+# To get the EOLMARK behavior, simply adjust the COLUMNS+1 expression to
+# account for the width of the mark, and include it.  For example:
+
+: setopt nopromptcr
+: PS1="%{%S<EOL>%s${(pl:COLUMNS-4:: ::\r:)}%}$PS1"
+
+# The important bit is that the total width of the string inside %{...%}
+# has to be COLUMNS+1, where the extra character is the \r.  However, I
+# recommend using a one-character EOLMARK to avoid having the line wrap
+# in the middle of the marker string:
+
+setopt nopromptcr
+PS1="%{%S#%s${(pl:COLUMNS:: ::\r:)}%}$PS1"