diff -uNr a/yrc/manifest b/yrc/manifest
--- a/yrc/manifest e2db0fe5b97891eb86eb3421146dd0505861fa5d457a129c6a0deae8a838787ec6b436a901fbbb67cee946ee128b2bd57da05414dc65a2f95f8b76a9efd747cd
+++ b/yrc/manifest cb588cedfb5c3350df728e539d79692b933cef15f3ff4670b4a1253cf9b38b35b9acfbfc9fb4b3ae5ec18a3126062b77e7d3083adf5925ec480b3356871bd934
@@ -4,3 +4,4 @@
 768773 yrc_minor_command_reorder jfw Move prompt_backspace and prompt_delete implementations to match the more sensible order in keymap and manual.
 768773 yrc_input_history jfw Implement input history, by use of a new data structure for rings (cyclic lists).
 768775 yrc_kill_yank jfw Implement the basic kill and yank commands. Also: rename prompt-backward command to prompt-back, to avert confusion with the anticipated prompt-back-word; update manual for the new commands plus clarifications & updated priorities.
+768776 yrc_prompt_redraw_optimization jfw Optimize and clarify prompt drawing code; remove unneeded prompt_set_hscroll for consistency (this turned out to be needed after all; see an upcoming patch for fix and explanation).
diff -uNr a/yrc/yrc.py b/yrc/yrc.py
--- a/yrc/yrc.py 59b7e716e692e29ef4e5a39ed30eb333bb6b6e225c8e55210eb627ed7cbb6ad671f9e9c2475ddf6bbde41bb12739156439e03920ae8f28ba91d2d657750e5eb5
+++ b/yrc/yrc.py 431ad816f70f17cbc8eb50be23683af4c97a38e9bb5597d2dd6bc7b47ad5f087fe09a75682c0d92846145afb776083de91ed38e7088ce0ba244cbd4b398d576a
@@ -850,6 +850,7 @@
 	return False
 
 def prompt_set_hscroll(s):
+	# This is called automatically by prompt_draw itself, but make sure a redraw gets done in case more calls are added.
 	schedule_prompt_draw()
 	prompt[2] = s
 
@@ -860,7 +861,7 @@
 	schedule_prompt_draw()
 	del prompt_chars()[:]
 	prompt_set_cursor(0)
-	prompt_set_hscroll(0)
+	# hscroll and cursor_column are always updated at drawing time so no need to explicitly reset them, as with any code that changes chars or cursor.
 
 history_ring = new_ring(HISTORY_RING_SIZE)
 history_pos = None
@@ -1045,22 +1046,26 @@
 	if scr_width < 4:
 		write_out(t.ERASE_LINE_TO_END)
 		return
+	# XXX Sometimes this redraws more than strictly necessary, such as when simply appending a character or moving the cursor without scrolling. Over a network the added transmission overhead is minimal (probably all fitting in the same packet as the required part of the update) but over a serial line it might be bothersome. A brute-force approach would be to stash the last rendered box contents, find the common prefix and draw only the rest.
 	prompt_str = '> '
 	write_out(prompt_str)
 	chars = prompt_chars()
 	cursor = prompt_cursor()
 	hscr = prompt_hscroll()
+	# XXX O(N) rendering (one difficulty is that hscroll is measured in rendered rather than source characters)
 	before_cursor = asciify(chars[:cursor])
 	cursor_offset = len(before_cursor) - hscr
 	max_width = scr_width - len(prompt_str)
 	if not 0 <= cursor_offset < max_width:
-		hscr = max(0, hscr + cursor_offset - max_width/2)
+		# Cursor outside box: scroll text to center it.
+		hscr = max(0, len(before_cursor) - max_width/2)
 		prompt_set_hscroll(hscr)
 		cursor_offset = len(before_cursor) - hscr
 	prompt_set_cursor_column(1 + len(prompt_str) + cursor_offset)
-	printable = (before_cursor[hscr:] +
-		asciify(chars[cursor:])[:max_width - cursor_offset])
-	write_out(printable)
+	write_out(before_cursor[hscr:])
+	trailing_cols = max_width - cursor_offset
+	# Pre-clip to skip needless O(N) rendering, then re-clip in case the rendering added characters.
+	write_out(asciify(chars[cursor:cursor+trailing_cols])[:trailing_cols])
 	write_out(t.ERASE_LINE_TO_END)
 	schedule_refresh()
 	prompt_draw_done()