text: fix zero-width truncated newline rune accounting

This commit fixes another rune accounting issue that only existed when shaping
a solitary newline with zero width while truncating the line.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

text/gotext.go

@@ -523,22 +523,27 @@ func calculateYOffsets(lines []line) {
 // LayoutRunes shapes and wraps the text, and returns the result in Gio's shaped text format.
 func (s *shaperImpl) LayoutRunes(params Parameters, txt []rune) document {
 	hasNewline := len(txt) > 0 && txt[len(txt)-1] == '\n'
-	justNewline := false
+	var ls []shaping.Line
+	var truncated int
 	if hasNewline {
 		txt = txt[:len(txt)-1]
-		if len(txt) == 0 {
-			// If we only have a newline, shape a space to get line metrics.
-			txt = []rune{' '}
-			justNewline = true
-		}
 	}
-	ls, truncated := s.shapeAndWrapText(params, replaceControlCharacters(txt))
-	if justNewline {
-		// We shaped a space to get proper line metrics, but we need to drop
-		// the rune/glyph info since it isn't actually part of the text.
-		ls[0][0].Glyphs = ls[0][0].Glyphs[:0]
-		ls[0][0].Runes.Count = 0
-		ls[0][0].Advance = 0
+	if hasNewline && len(txt) == 0 {
+		// If we only have a newline, shape a space to get line metrics.
+		ls, truncated = s.shapeAndWrapText(params, []rune{' '})
+		if truncated > 0 {
+			// Our space was truncated. Since our space didn't exist in any meaningful
+			// capacity, ensure the truncated count is zeroed out.
+			truncated = 0
+		} else {
+			// We shaped a space to get proper line metrics, but we need to drop
+			// the rune/glyph info since it isn't actually part of the text.
+			ls[0][0].Glyphs = ls[0][0].Glyphs[:0]
+			ls[0][0].Advance = 0
+			ls[0][0].Runes.Count = 0
+		}
+	} else {
+		ls, truncated = s.shapeAndWrapText(params, replaceControlCharacters(txt))
 	}
 
 	didTruncate := truncated > 0 || (params.forceTruncate && params.MaxLines == len(ls))

text/shaper_test.go

@@ -484,6 +484,19 @@ func TestShapeStringRuneAccounting(t *testing.T) {
 				MaxWidth:   999929,
 			},
 		},
+		{
+			name:  "newline zero-width regression",
+			input: "\n",
+			params: Parameters{
+				Font:       font.Font{Typeface: "Go", Style: font.Regular, Weight: font.Normal},
+				Alignment:  Start,
+				PxPerEm:    768,
+				MaxLines:   1,
+				Truncator:  "\u200b",
+				WrapPolicy: WrapHeuristically,
+				MaxWidth:   0,
+			},
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			for _, setup := range []setup{