From bc5e61615bb007df858ef1f801b90a7ebc31d75a Mon Sep 17 00:00:00 2001
From: Tiago Peczenyj <tpeczenyj@weborama.com>
Date: Fri, 29 May 2026 16:18:45 +0200
Subject: [PATCH] refactor(ui): simplify truncPad via runewidth
 Truncate/FillRight

Replace truncPad's hand-rolled width-aware truncate-and-pad loop with
runewidth's built-in Truncate/FillRight helpers. go-runewidth is already
a direct dependency, so this adds no modules while removing the manual
width bookkeeping (ellipsis-cell reservation, wide-rune boundary
handling, padding).

Output is byte-for-byte identical: verified against the side-by-side
golden fixture, the unicode/wide-rune tests, and a differential check
over 21 inputs x 28 widths (incl. negatives, zero, CJK, tabs, emoji,
accents, and the wide-rune-straddling case).

This is also the resolution of the side-by-side library investigation:
the lightest alternative to lipgloss/x-cellbuf/reflow is no new library
at all, since color is applied after truncPad and ANSI-aware truncation
buys us nothing.

Closes #93

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 internal/ui/printer.go | 27 +++++----------------------
 1 file changed, 5 insertions(+), 22 deletions(-)

diff --git a/internal/ui/printer.go b/internal/ui/printer.go
index 84761a1..ea1ee96 100644
--- a/internal/ui/printer.go
+++ b/internal/ui/printer.go
@@ -380,28 +380,11 @@ func truncPad(s string, w int) string {
 	}
 	// expand tabs to 4 spaces for stable columns
 	s = strings.ReplaceAll(s, "\t", "    ")
-	runes := []rune(s)
-	if total := runewidth.StringWidth(s); total <= w {
-		return s + strings.Repeat(" ", w-total)
-	}
-	if w == 1 {
-		return "…"
-	}
-	// Keep the widest prefix that still leaves one cell for the ellipsis, then
-	// pad: a wide rune straddling the boundary can leave the prefix a cell short.
-	width := 0
-	var b strings.Builder
-	for _, r := range runes {
-		rw := runewidth.RuneWidth(r)
-		if width+rw+1 > w { // reserve one cell for "…"
-			break
-		}
-		b.WriteRune(r)
-		width += rw
-	}
-	b.WriteString("…")
-	width++
-	return b.String() + strings.Repeat(" ", w-width)
+	// runewidth.Truncate fits s into w cells, appending "…" (and reserving its
+	// cell) when it has to cut — including the wide-rune-straddling-the-boundary
+	// case. FillRight then right-pads the result to exactly w cells. Both measure
+	// in terminal cells, so CJK and other wide runes count as two.
+	return runewidth.FillRight(runewidth.Truncate(s, w, "…"), w)
 }
 
 // paint renders s in role's style when color is enabled, else returns s plain.