Render new preprompt with previous one in mind

This fixes #145 by calculating how many lines the previous preprompt
assumes in the current terminal window and either erases preprompt lines
that are no longer relevant or allocates more space for the new
preprompt.

pure.zsh

@@ -21,6 +21,7 @@
 # \e[1G => go to position 1 in terminal
 # \e8   => restore cursor position
 # \e[K  => clears everything after the cursor on the current line
+# \e[2K => clear everything on the current line
 
 
 # turns seconds into human readable time
@@ -133,18 +134,45 @@ prompt_pure_preprompt_render() {
 		[[ "${prompt_pure_last_preprompt}" != "${preprompt}" ]] || return
 
 		# calculate length of preprompt and store it locally in preprompt_length
-		integer preprompt_length
+		integer preprompt_length lines
 		prompt_pure_string_length_to_var "${preprompt}" "preprompt_length"
 
 		# calculate number of preprompt lines for redraw purposes
-		integer lines=$(( (preprompt_length - 1) / COLUMNS + 1 ))
+		(( lines = ( preprompt_length - 1 ) / COLUMNS + 1 ))
+
+		# calculate previous preprompt lines to figure out how the new preprompt should behave
+		integer last_preprompt_length last_lines
+		prompt_pure_string_length_to_var "${prompt_pure_last_preprompt}" "last_preprompt_length"
+		(( last_lines = ( last_preprompt_length - 1 ) / COLUMNS + 1 ))
+
+		# clr_prev_preprompt erases visual artifacts from previous preprompt
+		local clr_prev_preprompt
+		if (( last_lines > lines )); then
+			# move cursor up by last_lines, clear the line and move it down by one line
+			clr_prev_preprompt="\e[${last_lines}A\e[2K\e[1B"
+			while (( last_lines - lines > 1 )); do
+				# clear the line and move cursor down by one
+				clr_prev_preprompt+='\e[2K\e[1B'
+				(( last_lines-- ))
+			done
+
+			# move cursor into correct position for preprompt update
+			clr_prev_preprompt+="\e[${lines}B"
+		# create more space for preprompt if new preprompt has more lines than last
+		elif (( last_lines < lines )); then
+			# move cursor using newlines because ansi cursor movement can't push the cursor beyond the last line
+			printf $'\n'%.0s {1..$(( lines - last_lines ))}
+
+			# redraw the prompt since it has been moved by print
+			zle && zle .reset-prompt
+		fi
 
 		# disable clearing of line if last char of preprompt is last column of terminal
 		local clr='\e[K'
 		(( COLUMNS * lines == preprompt_length )) && clr=
 
 		# modify previous preprompt
-		print -Pn "\e7\e[${lines}A\e[1G${preprompt}${clr}\e8"
+		print -Pn "\e7${clr_prev_preprompt}\e[${lines}A\e[1G${preprompt}${clr}\e8"
 	fi
 
 	# store previous preprompt for comparison